@Part(08, Root="ada.mss") @Comment{$Date: 2000/08/30 00:23:09 $} @LabeledSection{Visibility Rules} @Comment{$Source: e:\\cvsroot/ARM/Source/08.mss,v $} @Comment{$Revision: 1.23 $} @begin{Intro} @redundant[The rules defining the scope of declarations and the rules defining which @nt{identifier}s, @nt{character_literal}s, and @nt{operator_symbol}s are visible at (or from) various places in the text of the program are described in this section. The formulation of these rules uses the notion of a declarative region. As explained in Section 3, a declaration declares a view of an entity and associates a defining name with that view. The view comprises an identification of the viewed entity, and possibly additional properties. A usage name denotes a declaration. It also denotes the view declared by that declaration, and denotes the entity of that view. Thus, two different usage names might denote two different views of the same entity; in this case they denote the same entity.] @begin{Honest} In some cases, a usage name that denotes a declaration does not denote the view declared by that declaration, nor the entity of that view, but instead denotes a view of the current instance of the entity, and denotes the current instance of the entity. This sometimes happens when the usage name occurs inside the declarative region of the declaration. @end{Honest} @end{Intro} @begin{DiffWord83} We no longer define the term @lquotes@;basic operation;@rquotes@; thus we no longer have to worry about the visibility of them. Since they were essentially always visible in Ada 83, this change has no effect. The reason for this change is that the definition in Ada 83 was confusing, and not quite correct, and we found it difficult to fix. For example, one wonders why an @nt{if_statement} was not a basic operation of type Boolean. For another example, one wonders what it meant for a basic operation to be @lquotes@;inherent in@rquotes@; something. Finally, this fixes the problem addressed by AI83-00027/07. @end{DiffWord83} @LabeledClause{Declarative Region} @begin{StaticSem} @leading@Defn2{Term=[declarative region], Sec=(of a construct)} For each of the following constructs, there is a portion of the program text called its @i{declarative region}, @Redundant[within which nested declarations can occur]: @begin(itemize) any declaration, other than that of an enumeration type, that is not a completion @Redundant[of a previous declaration]; a @nt{block_statement}; a @nt{loop_statement}; an @nt{accept_statement}; an @nt{exception_handler}. @end(itemize) @begin{Wide} @leading@;The declarative region includes the text of the construct together with additional text determined @Redundant[(recursively)], as follows: @end{Wide} @begin{Itemize} If a declaration is included, so is its completion, if any. If the declaration of a library unit @Redundant[(including Standard @em see @RefSecNum{Compilation Units - Library Units})] is included, so are the declarations of any child units @Redundant[(and their completions, by the previous rule)]. The child declarations occur after the declaration. If a @nt{body_stub} is included, so is the corresponding @nt{subunit}. If a @nt{type_declaration} is included, then so is a corresponding @nt{record_representation_clause}, if any. @begin{Reason} This is so that the @nt{component_declaration}s can be directly visible in the @nt{record_representation_clause}. @end{Reason} @end{Itemize} The declarative region of a declaration is also called the @i{declarative region} of any view or entity declared by the declaration. @begin{Reason} The constructs that have declarative regions are the constructs that can have declarations nested inside them. Nested declarations are declared in that declarative region. The one exception is for enumeration literals; although they are nested inside an enumeration type declaration, they behave as if they were declared at the same level as the type. @end{Reason} @begin{Honest} A declarative region does not include @nt{parent_unit_name}s. @end{Honest} @begin{Ramification} A declarative region does not include @nt{context_clause}s. @end{Ramification} @Defn{occur immediately within} @Defn{immediately within} @Defn2{Term=[within],Sec=(immediately)} @Defn{immediately enclosing} @Defn2{Term=[enclosing],Sec=(immediately)} A declaration occurs @i{immediately within} a declarative region if this region is the innermost declarative region that encloses the declaration (the @i{immediately enclosing} declarative region), not counting the declarative region (if any) associated with the declaration itself. @begin{Discussion} Don't confuse the declarative region of a declaration with the declarative region in which it immediately occurs. @end{Discussion} @redundant[@Defn{local to} A declaration is @i{local} to a declarative region if the declaration occurs immediately within the declarative region.] @redundant[An entity is @i{local} to a declarative region if the entity is declared by a declaration that is local to the declarative region.] @begin{Ramification} "Occurs immediately within" and "local to" are synonyms (when referring to declarations). Thus, @lquotes@;local to@rquotes@; applies to both declarations and entities, whereas @lquotes@;occurs immediately within@rquotes@; only applies to declarations. We use this term only informally; for cases where precision is required, we use the term "occurs immediately within", since it is less likely to cause confusion. @end{Ramification} @Defn{global to} A declaration is @i{global} to a declarative region if the declaration occurs immediately within another declarative region that encloses the declarative region. An entity is @i{global} to a declarative region if the entity is declared by a declaration that is global to the declarative region. @end{StaticSem} @begin{Notes} The children of a parent library unit are inside the parent's declarative region, even though they do not occur inside the parent's declaration or body. This implies that one can use (for example) "P.Q" to refer to a child of P whose defining name is Q, and that after "@key[use] P;" Q can refer (directly) to that child. As explained above and in @RefSec{Compilation Units - Library Units}, all library units are descendants of Standard, and so are contained in the declarative region of Standard. They are @i{not} inside the declaration or body of Standard, but they @i{are} inside its declarative region. For a declarative region that comes in multiple parts, the text of the declarative region does not contain any text that might appear between the parts. Thus, when a portion of a declarative region is said to extend from one place to another in the declarative region, the portion does not contain any text that might appear between the parts of the declarative region. @begin{Discussion} It is necessary for the things that have a declarative region to include anything that contains declarations (except for enumeration type declarations). This includes any declaration that has a profile (that is, @nt{subprogram_declaration}, @nt{subprogram_body}, @nt{entry_declaration}, @nt{subprogram_renaming_declaration}, @nt{formal_subprogram_declaration}, access-to-subprogram @nt{type_declaration}), anything that has a @nt{discriminant_part} (that is, various kinds of @nt{type_declaration}), anything that has a @nt{component_list} (that is, record @nt{type_declaration} and record extension @nt{type_declaration}), and finally the declarations of task and protected units and packages. @end{Discussion} @end{Notes} @begin{DiffWord83} @leading@;It was necessary to extend Ada 83's definition of declarative region to take the following Ada 95 features into account: @begin{Itemize} Child library units. Derived types/type extensions @em we need a declarative region for inherited components and also for new components. All the kinds of types that allow discriminants. Protected units. Entries that have bodies instead of accept statements. The @nt{choice_parameter_specification} of an @nt{exception_handler}. The formal parameters of access-to-subprogram types. Renamings-as-body. @end{Itemize} Discriminated and access-to-subprogram type declarations need a declarative region. Enumeration type declarations cannot have one, because you don't have to say "Color.Red" to refer to the literal Red of Color. For other type declarations, it doesn't really matter whether or not there is an associated declarative region, so for simplicity, we give one to all types except enumeration types. We now say that an @nt{accept_statement} has its own declarative region, rather than being part of the declarative region of the @nt{entry_declaration}, so that declarative regions are properly nested regions of text, so that it makes sense to talk about "inner declarative regions," and "...extends to the end of a declarative region." Inside an @nt{accept_statement}, the @nt{name} of one of the parameters denotes the @nt{parameter_specification} of the @nt{accept_statement}, not that of the @nt{entry_declaration}. If the @nt{accept_statement} is nested within a @nt{block_statement}, these @nt{parameter_specification}s can hide declarations of the @nt{block_statement}. The semantics of such cases was unclear in RM83. @begin{Honest} Unfortunately, we have the same problem for the entry name itself @em it should denote the @nt{accept_statement}, but @nt{accept_statement}s are not declarations. They should be, and they should hide the entry from all visibility within themselves. @end{Honest} Note that we can't generalize this to @nt{entry_bodies}, or other bodies, because the @nt{declarative_part} of a body is not supposed to contain (explicit) homographs of things in the declaration. It works for @nt{accept_statement}s only because an @nt{accept_statement} does not have a @nt{declarative_part}. To avoid confusion, we use the term @lquotes@;local to@rquotes@; only informally in Ada 95. Even RM83 used the term incorrectly (see, for example, RM83-12.3(13)). In Ada 83, (root) library units were inside Standard; it was not clear whether the declaration or body of Standard was meant. In Ada 95, they are children of Standard, and so occur immediately within Standard's declarative region, but not within either the declaration or the body. (See RM83-8.6(2) and RM83-10.1.1(5).) @end{DiffWord83} @LabeledClause{Scope of Declarations} @begin{Intro} @redundant[For each declaration, the language rules define a certain portion of the program text called the @i{scope} of the declaration. The scope of a declaration is also called the scope of any view or entity declared by the declaration. Within the scope of an entity, and only there, there are places where it is legal to refer to the declared entity. These places are defined by the rules of visibility and overloading.] @end{Intro} @begin{StaticSem} @Leading@Defn2{Term=[immediate scope], Sec=(of a declaration)} The @i{immediate scope} of a declaration is a portion of the declarative region immediately enclosing the declaration. The immediate scope starts at the beginning of the declaration, except in the case of an overloadable declaration, in which case the immediate scope starts just after the place where the profile of the callable entity is determined (which is at the end of the @nt<_specification> for the callable entity, or at the end of the @nt if an instance). The immediate scope extends to the end of the declarative region, with the following exceptions: @begin{Reason} The reason for making overloadable declarations with profiles special is to simplify compilation: until the compiler has determined the profile, it doesn't know which other declarations are homographs of this one, so it doesn't know which ones this one should hide. Without this rule, two passes over the @nt<_specification> or @nt would be required to resolve names that denote things with the same name as this one. @end{Reason} @begin{Itemize} The immediate scope of a @nt{library_item} includes only its semantic dependents. @begin{Reason} Section 10 defines only a partial ordering of @nt{library_item}s. Therefore, it is a good idea to restrict the immediate scope (and the scope, defined below) to semantic dependents. @leading@;Consider also examples like this: @begin{Example} @key[package] P @key[is] @key[end] P; @key[package] P.Q @key[is] I : Integer := 0; @key[end] P.Q; @key[with] P; @key[package] R @key[is] @key[package] X @key[renames] P; X.Q.I := 17; --@RI{ Illegal!} @key[end] R; @end{Example} The scope of P.Q does not contain R. Hence, neither P.Q nor X.Q are visible within R. However, the name R.X.Q would be visible in some other library unit where both R and P.Q are visible (assuming R were made legal by removing the offending declaration). @end{Reason} The immediate scope of a declaration in the private part of a library unit does not include the visible part of any public descendant of that library unit. @Pdefn2{Term=[descendant],Sec=(relationship with scope)} @begin{Ramification} In other words, a declaration in the private part can be visible within the visible part, private part and body of a private child unit. On the other hand, such a declaration can be visible within only the private part and body of a public child unit. @end{Ramification} @begin{Reason} The purpose of this rule is to prevent children from giving private information to clients. @end{Reason} @begin{Ramification} For a public child subprogram, this means that the parent's private part is not visible in the @nt{formal_part}s of the declaration and of the body. This is true even for @nt{subprogram_bodies} that are not completions. For a public child generic unit, it means that the parent's private part is not visible in the @nt{generic_formal_part}, as well as in the first list of @nt{basic_declarative_item}s (for a generic package), or the @nt{formal_part}(s) (for a generic subprogram). @end{Ramification} @end{Itemize} @Defn{visible part} @Redundant[The @i(visible part) of (a view of) an entity is a portion of the text of its declaration containing declarations that are visible from outside.] @RootDefn{private part} The @i{private part} of (a view of) an entity that has a visible part contains all declarations within the declaration of (the view of) the entity, except those in the visible part; @Redundant[these are not visible from outside. Visible and private parts are defined only for these kinds of entities: callable entities, other program units, and composite types.] @begin{Itemize} @PDefn2{Term=[visible part], Sec=(of a view of a callable entity)} The visible part of a view of a callable entity is its profile. @PDefn2{Term=[visible part], Sec=(of a view of a composite type)} The visible part of a composite type other than a task or protected type consists of the declarations of all components declared @Redundant[(explicitly or implicitly)] within the @nt{type_declaration}. @PDefn2{Term=[visible part], Sec=(of a generic unit)} The visible part of a generic unit includes the @nt{generic_formal_part}. For a generic package, it also includes the first list of @nt{basic_declarative_item}s of the @nt{package_specification}. For a generic subprogram, it also includes the profile. @begin{Reason} Although there is no way to reference anything but the formals from outside a generic unit, they are still in the visible part in the sense that the corresponding declarations in an instance can be referenced (at least in some cases). In other words, these declarations have an effect on the outside world. The visible part of a generic unit needs to be defined this way in order to properly support the rule that makes a parent's private part invisible within a public child's visible part. @end{Reason} @begin{Ramification} The visible part of an instance of a generic unit is as defined for packages and subprograms; it is not defined in terms of the visible part of a generic unit. @end{Ramification} @Redundant[The visible part of a package, task unit, or protected unit consists of declarations in the program unit's declaration other than those following the reserved word @key{private}, if any; see @RefSecNum{Package Specifications and Declarations} and @RefSecNum{Formal Packages} for packages, @RefSecNum{Task Units and Task Objects} for task units, and @RefSecNum{Protected Units and Protected Objects} for protected units.] @end{Itemize} @Defn2{Term=[scope], Sec=(of a declaration)} The scope of a declaration always contains the immediate scope of the declaration. In addition, for a given declaration that occurs immediately within the visible part of an outer declaration, or is a public child of an outer declaration, the scope of the given declaration extends to the end of the scope of the outer declaration, except that the scope of a @nt{library_item} includes only its semantic dependents. @begin{Ramification} Note the recursion. If a declaration appears in the visible part of a library unit, its scope extends to the end of the scope of the library unit, but since that only includes dependents of the declaration of the library unit, the scope of the inner declaration also only includes those dependents. If X renames library package P, which has a child Q, a @nt{with_clause} mentioning P.Q is necessary to be able to refer to X.Q, even if P.Q is visible at the place where X is declared. @end{Ramification} @Defn2{Term=[immediate scope], Sec=[of (a view of) an entity]} The immediate scope of a declaration is also the immediate scope of the entity or view declared by the declaration. @Defn2{Term=[scope], Sec=[of (a view of) an entity]} Similarly, the scope of a declaration is also the scope of the entity or view declared by the declaration. @begin{Ramification} @leading@;The rule for immediate scope implies the following: @begin{Itemize} If the declaration is that of a library unit, then the immediate scope includes the declarative region of the declaration itself, but not other places, unless they are within the scope of a @nt{with_clause} that mentions the library unit. @NoPrefix@;It is necessary to attach the semantics of @nt{with_clause}s to [immediate] scopes (as opposed to visibility), in order for various rules to work properly. A library unit should hide a homographic implicit declaration that appears in its parent, but only within the scope of a @nt{with_clause} that mentions the library unit. Otherwise, we would violate the "legality determinable via semantic dependences" rule of @RefSec{Program Structure and Compilation Issues}. The declaration of a library unit should be allowed to be a homograph of an explicit declaration in its parent's body, so long as that body does not mention the library unit in a @nt{with_clause}. @NoPrefix@;This means that one cannot denote the declaration of the library unit, but one might still be able to denote the library unit via another view. @NoPrefix@;A @nt{with_clause} does not make the declaration of a library unit visible; the lack of a @nt{with_clause} prevents it from being visible. Even if a library unit is mentioned in a @nt{with_clause}, its declaration can still be hidden. The completion of the declaration of a library unit (assuming that's also a declaration) is not visible, neither directly nor by selection, outside that completion. The immediate scope of a declaration immediately within the body of a library unit does not include any child of that library unit. @NoPrefix@;This is needed to prevent children from looking inside their parent's body. The children are in the declarative region of the parent, and they might be after the parent's body. Therefore, the scope of a declaration that occurs immediately within the body might include some children. @end{Itemize} @end{Ramification} @end{StaticSem} @begin{Notes} There are notations for denoting visible declarations that are not directly visible. For example, @nt{parameter_@!specification}s are in the visible part of a @nt{subprogram_@!declaration} so that they can be used in named-notation calls appearing outside the called subprogram. For another example, declarations of the visible part of a package can be denoted by expanded names appearing outside the package, and can be made directly visible by a @nt{use_clause}. @begin{Ramification} There are some obscure involving generics cases in which there is no such notation. See Section 12. @end{Ramification} @end{Notes} @begin{Extend83} The fact that the immediate scope of an overloadable declaration does not include its profile is new to Ada 95. It replaces RM83-8.3(16), which said that within a subprogram specification and within the formal part of an entry declaration or accept statement, all declarations with the same designator as the subprogram or entry were hidden from all visibility. The RM83-8.3(16) rule seemed to be overkill, and created both implementation difficulties and unnecessary semantic complexity. @end{Extend83} @begin{DiffWord83} We no longer need to talk about the scope of notations, @nt{identifier}s, @nt{character_literal}s, and @nt{operator_symbol}s. The notion of "visible part" has been extended in Ada 95. The syntax of task and protected units now allows private parts, thus requiring us to be able to talk about the visible part as well. It was necessary to extend the concept to subprograms and to generic units, in order for the visibility rules related to child library units to work properly. It was necessary to define the concept separately for generic formal packages, since their visible part is slightly different from that of a normal package. Extending the concept to composite types made the definition of scope slightly simpler. We define visible part for some things elsewhere, since it makes a big difference to the user for those things. For composite types and subprograms, however, the concept is used only in arcane visibility rules, so we localize it to this clause. In Ada 83, the semantics of @nt{with_clause}s was described in terms of visibility. It is now described in terms of [immediate] scope. @leading@;We have clarified that the following is illegal (where Q and R are library units): @begin{Example} @key[package] Q @key[is] I : Integer := 0; @key[end] Q; @key[package] R @key[is] @key[package] X @key[renames] Standard; X.Q.I := 17; --@RI{ Illegal!} @key[end] R; @end{Example} even though Q is declared in the declarative region of Standard, because R does not mention Q in a @nt{with_clause}. @end{DiffWord83} @LabeledClause{Visibility} @begin{Intro} @redundant[@Defn{visibility rules} The @i{visibility rules}, given below, determine which declarations are visible and directly visible at each place within a program. The visibility rules apply to both explicit and implicit declarations.] @end{Intro} @begin{StaticSem} @Defn2{Term=[visibility], Sec=(direct)} @Defn{directly visible} @Defn{directly visible} A declaration is defined to be @i{directly visible} at places where a @nt consisting of only an @nt{identifier} or @nt{operator_symbol} is sufficient to denote the declaration; that is, no @nt notation or special context (such as preceding => in a named association) is necessary to denote the declaration. @Defn{visible} A declaration is defined to be @i{visible} wherever it is directly visible, as well as at other places where some @nt (such as a @nt) can denote the declaration. The syntactic category @nt is used to indicate contexts where direct visibility is required. The syntactic category @nt is used to indicate contexts where visibility, but not direct visibility, is required. @Defn2{Term=[visibility], Sec=(immediate)} @Defn2{Term=[visibility], Sec=(use clause)} There are two kinds of direct visibility: @i{immediate visibility} and @i{use-visibility}. @Defn{immediately visible} A declaration is immediately visible at a place if it is directly visible because the place is within its immediate scope. @Defn{use-visible} A declaration is use-visible if it is directly visible because of a @nt{use_clause} (see @RefSecNum{Use Clauses}). Both conditions can apply. @Defn{hiding} A declaration can be @i{hidden}, either from direct visibility, or from all visibility, within certain parts of its scope. @Defn{hidden from all visibility} Where @i{hidden from all visibility}, it is not visible at all (neither using a @nt nor a @nt). @Defn{hidden from direct visibility} Where @i{hidden from direct visibility}, only direct visibility is lost; visibility using a @nt is still possible. @redundant[@Defn{overloaded} Two or more declarations are @i{overloaded} if they all have the same defining name and there is a place where they are all directly visible.] @begin{Ramification} Note that a @nt{name} can have more than one possible interpretation even if it denotes a non-overloadable entity. For example, if there are two functions F that return records, both containing a component called C, then the name F.C has two possible interpretations, even though component declarations are not overloadable. @end{Ramification} @Defn{overloadable} The declarations of callable entities @Redundant[(including enumeration literals)] are @i{overloadable}@Redundant[, meaning that overloading is allowed for them]. @begin{Ramification} A @nt{generic_declaration} is not overloadable within its own @nt{generic_formal_part}. This follows from the rules about when a @nt{name} denotes a current instance. See AI83-00286. This implies that within a @nt{generic_formal_part}, outer declarations with the same defining name are hidden from direct visibility. It also implies that if a generic formal parameter has the same defining name as the generic itself, the formal parameter hides the generic from direct visibility. @end{Ramification} @Defn{homograph} Two declarations are @i{homographs} if they have the same defining name, and, if both are overloadable, their profiles are type conformant. @PDefn{type conformance} @redundant[An inner declaration hides any outer homograph from direct visibility.] @ChgRef{Version=[1],Kind=[Revised],Ref=[8652/0025]} @leading@Redundant[Two homographs are not generally allowed immediately within the same declarative region unless one @i{overrides} the other (see Legality Rules below).] @Defn{override} @Chg{New=[The only declarations that are @Defn{overridable}@i{overridable} are the implicit declarations for predefined operators and inherited primitive subprograms.],Old=[]} A declaration overrides another homograph that occurs immediately within the same declarative region in the following cases: @begin{Itemize} @ChgRef{Version=[1],Kind=[Revised],Ref=[8652/0025]} @Chg{New=[A declaration that is not overridable overrides one that is overridable], Old=[An explicit declaration overrides an implicit declaration of a primitive subprogram]}, @Redundant[regardless of which declaration occurs first]; @begin{Ramification} @ChgRef{Version=[1],Kind=[Revised],Ref=[8652/0025]} And regardless of whether the @Chg{New=[non-overriddable],Old=[explicit]} declaration is overloadable or not. @Chg{New=[For example, @nt{statement_identifier}s are covered by this rule.],Old=[]} The @lquotes@;regardless of which declaration occurs first@rquotes@; is there because the explicit declaration could be a primitive subprogram of a partial view, and then the full view might inherit a homograph. We are saying that the explicit one wins (within its scope), even though the implicit one comes later. If the overriding declaration is also a subprogram, then it is a primitive subprogram. As explained in @RefSec{Private Operations}, some inherited primitive subprograms are never declared. Such subprograms cannot be overridden, although they can be reached by dispatching calls in the case of a tagged type. @end{Ramification} The implicit declaration of an inherited operator overrides that of a predefined operator; @begin{Ramification} In a previous version of Ada 9X, we tried to avoid the notion of predefined operators, and say that they were inherited from some magical root type. However, this seemed like too much mechanism. Therefore, a type can have a predefined "+" as well as an inherited "+". The above rule says the inherited one wins. The @lquotes@;regardless of which declaration occurs first@rquotes@; applies here as well, in the case where @nt{derived_type_declaration} in the visible part of a public library unit derives from a private type declared in the parent unit, and the full view of the parent type has additional predefined operators, as explained in @RefSec{Private Operations}. Those predefined operators can be overridden by inherited subprograms implicitly declared earlier. @end{Ramification} An implicit declaration of an inherited subprogram overrides a previous implicit declaration of an inherited subprogram. @Redundant[For an implicit declaration of a primitive subprogram in a generic unit, there is a copy of this declaration in an instance.] However, a whole new set of primitive subprograms is implicitly declared for each type declared within the visible part of the instance. These new declarations occur immediately after the type declaration, and override the copied ones. @Redundant[The copied ones can be called only from within the instance; the new ones can be called only from outside the instance, although for tagged types, the body of a new one can be executed by a call to an old one.] @begin{Discussion} In addition, this is also stated redundantly (again), and is repeated, in @RefSec{Generic Instantiation}. The rationale for the rule is explained there. @end{Discussion} @end{Itemize} @leading@Defn{visible} @RootDefn{hidden from all visibility} A declaration is visible within its scope, except where hidden from all visibility, as follows: @begin{Itemize} @PDefn2{Term=[hidden from all visibility], Sec=(for overridden declaration)} An overridden declaration is hidden from all visibility within the scope of the overriding declaration. @begin{Ramification} We have to talk about the scope of the overriding declaration, not its visibility, because it hides even when it is itself hidden. Note that the scope of an explicit @nt{subprogram_declaration} does not start until after its profile. @end{Ramification} @leading@PDefn2{Term=[hidden from all visibility], Sec=(within the declaration itself)} A declaration is hidden from all visibility until the end of the declaration, except: @begin(InnerItemize) For a record type or record extension, the declaration is hidden from all visibility only until the reserved word @b(record); For a @nt{package_declaration}, task declaration, protected declaration, @nt{generic_@!package_@!declaration}, or @nt{subprogram_@!body}, the declaration is hidden from all visibility only until the reserved word @b(is) of the declaration. @begin{Ramification} We're talking about the @key{is} of the construct itself, here, not some random @key{is} that might appear in a @nt{generic_formal_part}. @end{Ramification} @end(InnerItemize) @PDefn2{Term=[hidden from all visibility], Sec=(for a declaration completed by a subsequent declaration)} If the completion of a declaration is a declaration, then within the scope of the completion, the first declaration is hidden from all visibility. Similarly, a @nt{discriminant_@!specification} or @nt{parameter_@!specification} is hidden within the scope of a corresponding @nt{discriminant_@!specification} or @nt{parameter_@!specification} of a corresponding completion, or of a corresponding @nt{accept_@!statement}. @begin{Ramification} This rule means, for example, that within the scope of a @nt{full_type_declaration} that completes a @nt{private_type_declaration}, the name of the type will denote the @nt{full_type_declaration}, and therefore the full view of the type. On the other hand, if the completion is not a declaration, then it doesn't hide anything, and you can't denote it. @end{Ramification} @PDefn2{Term=[hidden from all visibility], Sec=(by lack of a @nt{with_clause})} The declaration of a library unit (including a @nt{library_unit_renaming_declaration}) is hidden from all visibility except at places that are within its declarative region or within the scope of a @nt{with_clause} that mentions it. @Redundant[For each declaration or renaming of a generic unit as a child of some parent generic package, there is a corresponding declaration nested immediately within each instance of the parent.] Such a nested declaration is hidden from all visibility except at places that are within the scope of a @nt{with_clause} that mentions the child. @begin{Discussion} This is the rule that prevents @nt{with_clause}s from being transitive; the [immediate] scope includes indirect semantic dependents. @end{Discussion} @end{Itemize} @leading@Defn{directly visible} @Defn{immediately visible} @Defn2{Term=[visibility], Sec=(direct)} @Defn2{Term=[visibility], Sec=(immediate)} A declaration with a @nt{defining_identifier} or @nt{defining_operator_symbol} is immediately visible @Redundant[(and hence directly visible)] within its immediate scope @RootDefn{hidden from direct visibility} except where hidden from direct visibility, as follows: @begin{Itemize} @PDefn2{Term=[hidden from direct visibility], Sec=(by an inner homograph)} A declaration is hidden from direct visibility within the immediate scope of a homograph of the declaration, if the homograph occurs within an inner declarative region; @PDefn2{Term=[hidden from direct visibility], Sec=(where hidden from all visibility)} A declaration is also hidden from direct visibility where hidden from all visibility. @end{Itemize} @end{StaticSem} @begin{Resolution} @PDefn2{Term=[possible interpretation], Sec=(for @nt{direct_name}s)} A @nt{direct_name} shall resolve to denote a directly visible declaration whose defining name is the same as the @nt{direct_name}. @PDefn2{Term=[possible interpretation], Sec=(for @nt{selector_name}s)} A @nt{selector_name} shall resolve to denote a visible declaration whose defining name is the same as the @nt{selector_name}. @begin{Discussion} "The same as" has the obvious meaning here, so for +, the possible interpretations are declarations whose defining name is "+" (an @nt{operator_symbol}). @end{Discussion} These rules on visibility and direct visibility do not apply in a @nt{context_clause}, a @nt{parent_unit_name}, or a @nt{pragma} that appears at the place of a @nt{compilation_unit}. For those contexts, see the rules in @RefSec{Environment-Level Visibility Rules}. @begin{Ramification} Direct visibility is irrelevant for @nt{character_literal}s. In terms of overload resolution @nt{character_literal}s are similar to other literals, like @key{null} @em see @RefSecNum{Literals}. For @nt{character_literal}s, there is no need to worry about hiding, since there is no way to declare homographs. @end{Ramification} @end{Resolution} @begin{Legality} @ChgRef{Version=[1],Kind=[Revised],Ref=[8652/0025],Ref=[8652/0026]} @Chg{New=[A non-overridable],Old=[An explicit]} declaration is illegal if there is a homograph occurring immediately within the same declarative region that is visible at the place of the declaration, and is not hidden from all visibility by the @Chg{New=[non-overridable],Old=[explicit]} declaration. @Chg{New=[In addition, a type extension is illegal if somewhere within its immediate scope it has two visible components with the same name.],Old=[]} Similarly, the @nt for a @nt is illegal if it mentions (in a @nt) some library unit, and there is a homograph of the library unit that is visible at the place of the corresponding stub, and the homograph and the mentioned library unit are both declared immediately within the same declarative region. @PDefn{generic contract issue} These rules also apply to dispatching operations declared in the visible part of an instance of a generic unit. However, they do not apply to other overloadable declarations in an instance@Redundant[; such declarations may have type conformant profiles in the instance, so long as the corresponding declarations in the generic were not type conformant]. @PDefn{type conformance} @begin{Discussion} @leading@;Normally, these rules just mean you can't explicitly declare two homographs immediately within the same declarative region. The wording is designed to handle the following special cases: @begin{Itemize} If the second declaration completes the first one, the second declaration is legal. @leading@;If the body of a library unit contains an explicit homograph of a child of that same library unit, this is illegal only if the body mentions the child in its @nt, or if some subunit mentions the child. Here's an example: @begin{Example} @key[package] P @key[is] @key[end] P; @key[package] P.Q @key[is] @key[end] P.Q; @key[package] @key[body] P @key[is] Q : Integer; --@RI{ OK; we cannot see package P.Q here.} @key[procedure] Sub @key[is] @key[separate]; @key[end] P; @key[with] P.Q; @key[separate](P) @key[procedure] Sub @key[is] --@RI{ Illegal.} @key[begin] @key[null]; @key[end] Sub; @end{Example} @NoPrefix@;If package body P said "@key[with] P.Q;", then it would be illegal to declare the homograph Q: Integer. But it does not, so the body of P is OK. However, the subunit would be able to see both P.Q's, and is therefore illegal. @NoPrefix@;A previous version of Ada 9X allowed the subunit, and said that references to P.Q would tend to be ambiguous. However, that was a bad idea, because it requires overload resolution to resolve references to directly visible non-overloadable homographs, which is something compilers have never before been required to do. @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0026],Ref=[8652/0102]} @Chg{New=[@Leading@;If a @nt{type_extension} contains a component with the same name as a complete in an ancestor type, there must be no place where both components are visible. For instance:],Old=[]} @begin{Example}@ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key[package] A @key[is] @key[type] T @key[is tagged private]; @key[package] B @key[is] @key[type] NT @key[is new] T @key[with record] I: Integer; -- @RI{Illegal because T.I is visible in the body.} @key[end record]; -- @RI{T.I is not visible here.} @key[end] B; @key[private] @key[type] T @key[is tagged record] I: Integer; -- @RI{Illegal because T.I is visible in the body.} @key[end record]; @key[end] A;],Old=[]} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key[package] A @key[is] @key[package] @key[body] A @key[is] @key[package] @key[body] B @key[is] -- @RI{T.I becomes visible here.} @key[end] B; @key[end] A;],Old=[]} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key[package] A.C @key[is] @key[type] NT2 @key[is new] A.T @key[with record] I: Integer; -- @RI{Illegal because T.I is visible in the private part.} @key[end record]; -- @RI{T.I is not visible here.} @key[private] -- @RI{T.I is visible here.} @key[end] A.C;],Old=[]} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key[with] A; @key[package] D @key[is] @key[type] NT3 @key[is new] A.T @key[with record] I: Integer; -- @RI{Legal because T.I is never visible in this package.} @key[end record]; @key[end] D;],Old=[]} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key[with] D; @key[package] A.E @key[is] @key[type] NT4 @key[is new] D.NT3 @key[with null record]; X : NT4; I1 : Integer := X.I; -- @RI{D.NT3.I} I2 : Integer := D.NT3(X).I; -- @RI{D.NT3.I} I3 : Integer := A.T(X).I; -- @RI{A.T.I} @key[end] A.E;],Old=[]} @end{Example} @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0102]} @Chg{New=[@Noprefix@;D.NT3 can have a component I because the component I of the parent type is never visible. The parent component exists, of course, but is never declared for the type D.NT3. In the child package A.E, the component I of A.T is visible, but that does not change the fact that the A.T.I component was never declared for type D.NT3. Thus, A.E.NT4 does not (visibly) inherit the component I from A.T, while it does inherit the component I from D.NT3. Of course, both components exist, and can be accessed by a type conversion as shown above. This behavior stems from the fact that every characteristic of a type (including components) must be declared somewhere in the innermost declarative region containing the type - if the characteristic is never visible in that declarative region, it is not inherited or declared. Therefore, such characteristics do not suddenly become available even if they are in fact visible in some other scope. See @RefSecNum{Private Operations} for more on the rules.],Old=[]} @end{Itemize} Note that we need to be careful which things we make "hidden from all visibility" versus which things we make simply illegal for names to denote. The distinction is subtle. The rules that disallow names denoting components within a type declaration (see @RefSecNum{Discriminants}) do not make the components invisible at those places, so that the above rule makes components with the same name illegal. The same is true for the rule that disallows names denoting formal parameters within a @nt{formal_part} (see @RefSecNum{Subprogram Declarations}). @end{Discussion} @begin{Discussion} The part about instances is from AI83-00012. The reason it says @lquotes@;overloadable declarations@rquotes@; is because we don't want it to apply to type extensions that appear in an instance; components are not overloadable. @end{Discussion} @end{Legality} @begin{Notes} Visibility for compilation units follows from the definition of the environment in @RefSecNum{The Compilation Process}, except that it is necessary to apply a @nt{with_clause} to obtain visibility to a @nt{library_unit_declaration} or @nt{library_unit_renaming_declaration}. In addition to the visibility rules given above, the meaning of the occurrence of a @nt{direct_name} or @nt{selector_name} at a given place in the text can depend on the overloading rules (see @RefSecNum{The Context of Overload Resolution}). Not all contexts where an @nt, @nt, or @nt are allowed require visibility of a corresponding declaration. Contexts where visibility is not required are identified by using one of these three syntactic categories directly in a syntax rule, rather than using @nt or @nt. @begin{Ramification} @leading@;An @nt{identifier}, @nt{character_literal} or @nt{operator_symbol} that occurs in one of the following contexts is not required to denote a visible or directly visible declaration: @begin{enumerate} A defining name. The @nt{identifier}s or @nt{operator_symbol} that appear after the reserved word @key{end} in a @nt{proper_body}. Similarly for @lquotes@;@key{end loop}@rquotes@;, etc. An @nt{attribute_designator}. A @nt{pragma} @nt{identifier}. A @SynI{pragma_argument_}@nt{identifier}. An @nt{identifier} specific to a pragma used in a pragma argument. @end{enumerate} The visibility rules have nothing to do with the above cases; the meanings of such things are defined elsewhere. Reserved words are not @nt{identifier}s; the visibility rules don't apply to them either. Because of the way we have defined "declaration", it is possible for a usage name to denote a @nt{subprogram_body}, either within that body, or (for a non-library unit) after it (since the body hides the corresponding declaration, if any). Other bodies do not work that way. Completions of @nt{type_} and @nt{deferred_constant_declaration}s do work that way. @nt{Accept_statements} are never denoted, although the @nt{parameter_specification}s in their profiles can be. @leading@;The scope of a subprogram does not start until after its profile. Thus, the following is legal: @begin{Example} X : @key[constant] Integer := 17; ... @key[package] P @key[is] @key[procedure] X(Y : @key[in] Integer := X); @key[end] P; @end{Example} The body of the subprogram will probably be illegal, however, since the constant X will be hidden by then. @leading@;The rule is different for generic subprograms, since they are not overloadable; the following is illegal: @begin{Example} X : @key[constant] Integer := 17; @key[package] P @key[is] @key[generic] Z : Integer := X; --@RI{ Illegal!} @key[procedure] X(Y : @key[in] Integer := X); --@RI{ Illegal!} @key[end] P; @end{Example} The constant X is hidden from direct visibility by the generic declaration. @end{Ramification} @end{Notes} @begin{Extend83} Declarations with the same defining name as that of a subprogram or entry being defined are nevertheless visible within the subprogram specification or entry declaration. @end{Extend83} @begin{DiffWord83} The term @lquotes@;visible by selection@rquotes@; is no longer defined. We use the terms @lquotes@;directly visible@rquotes@; and @lquotes@;visible@rquotes@; (among other things). There are only two regions of text that are of interest, here: the region in which a declaration is visible, and the region in which it is directly visible. Visibility is defined only for declarations. @end{DiffWord83} @LabeledClause{Use Clauses} @begin{Intro} @redundant[A @nt{use_package_clause} achieves direct visibility of declarations that appear in the visible part of a package; a @nt{use_type_clause} achieves direct visibility of the primitive operators of a type.] @end{Intro} @begin{MetaRules} @Defn{equivalence of @nt{use_clause}s and @nt{selected_component}s} If and only if the visibility rules allow P.A, "@key[use] P;" should make A directly visible (barring name conflicts). This means, for example, that child library units, and generic formals of a formal package whose @nt{formal_package_actual_part} is (<>), should be made visible by a @nt{use_clause} for the appropriate package. @Defn{Beaujolais effect} The rules for @nt{use_clause}s were carefully constructed to avoid so-called @i(Beaujolais) effects, where the addition or removal of a single @nt{use_clause}, or a single declaration in a "use"d package, would change the meaning of a program from one legal interpretation to another. @end{MetaRules} @begin{Syntax} @Syn{lhs=,rhs="@Syn2{use_package_clause} | @Syn2{use_type_clause}"} @Syn{lhs=,rhs="@key{use} @SynI{package_}@Syn2{name} {, @SynI{package_}@Syn2{name}};"} @Syn{lhs=,rhs="@key{use type} @Syn2{subtype_mark} {, @Syn2{subtype_mark}};"} @end{Syntax} @begin{Legality} A @SynI{package_}@nt{name} of a @nt{use_package_clause} shall denote a package. @begin{Ramification} This includes formal packages. @end{Ramification} @end{Legality} @begin{StaticSem} @Defn2{Term=[scope], Sec=(of a @nt{use_clause})} For each @nt{use_clause}, there is a certain region of text called the @i{scope} of the @nt{use_clause}. For a @nt{use_clause} within a @nt{context_clause} of a @nt{library_unit_declaration} or @nt{library_unit_renaming_declaration}, the scope is the entire declarative region of the declaration. For a @nt{use_clause} within a @nt{context_clause} of a body, the scope is the entire body @Redundant[and any subunits (including multiply nested subunits). The scope does not include @nts themselves.] For a @nt{use_clause} immediately within a declarative region, the scope is the portion of the declarative region starting just after the @nt{use_clause} and extending to the end of the declarative region. However, the scope of a @nt{use_clause} in the private part of a library unit does not include the visible part of any public descendant of that library unit. @begin{Reason} @leading@;The exception echoes the similar exception for @lquotes@;immediate scope (of a declaration)@rquotes@; (see @RefSecNum{Scope of Declarations}). It makes @nt{use_clause}s work like this: @begin{Example} @key[package] P @key[is] @key[type] T @key[is] @key[range] 1..10; @key[end] P; @key[with] P; @key[package] Parent @key[is] @key[private] @key[use] P; X : T; @key[end] Parent; @key[package] Parent.Child @key[is] Y : T; --@RI{ Illegal!} Z : P.T; @key[private] W : T; @key[end] Parent.Child; @end{Example} The declaration of Y is illegal because the scope of the @lquotes@;@key[use] P@rquotes@; does not include that place, so T is not directly visible there. The declarations of X, Z, and W are legal. @end{Reason} @Defn{potentially use-visible} For each package denoted by a @SynI{package_}@nt{name} of a @nt{use_package_clause} whose scope encloses a place, each declaration that occurs immediately within the declarative region of the package is @i(potentially) @i(use-visible) at this place if the declaration is visible at this place. @Comment{potentially and use-visible get separate i commands, to prevent a Scribe bug that causes use-visible to not be italicized.} For each type @i(T) or @i(T)'Class determined by a @nt of a @nt{use_type_clause} whose scope encloses a place, the declaration of each primitive operator of type @i(T) is potentially use-visible at this place if its declaration is visible at this place. @begin{Ramification} Primitive subprograms whose defining name is an @nt{identifier} are @i{not} made potentially visible by a @nt{use_type_clause}. A @nt{use_type_clause} is only for operators. The semantics described here should be similar to the semantics for expanded names given in @RefSec{Selected Components} so as to achieve the effect requested by the @lquotes@;principle of equivalence of @nt{use_clause}s and @nt{selected_component}s.@rquotes@; Thus, child library units and generic formal parameters of a formal package are potentially use-visible when their enclosing package is use'd. The "visible at that place" part implies that applying a @nt{use_clause} to a parent unit does not make all of its children use-visible @em only those that have been made visible by a @nt{with_clause}. It also implies that we don't have to worry about hiding in the definition of "directly visible" @em a declaration cannot be use-visible unless it is visible. Note that "@key[use type] T'Class;" is equivalent to "@key[use type] T;", which helps avoid breaking the generic contract model. @end{Ramification} @leading@Defn{use-visible} @Defn2{Term=[visibility],Sec=(use clause)} A declaration is @i{use-visible} if it is potentially use-visible, except in these naming-conflict cases: @begin{itemize} A potentially use-visible declaration is not use-visible if the place considered is within the immediate scope of a homograph of the declaration. Potentially use-visible declarations that have the same @nt{identifier} are not use-visible unless each of them is an overloadable declaration. @begin{Ramification} Overloadable declarations don't cancel each other out, even if they are homographs, though if they are not distinguishable by formal parameter names or the presence or absence of @nt{default_expression}s, any use will be ambiguous. We only mention @nt{identifier}s here, because declarations named by @nts are always overloadable, and hence never cancel each other. Direct visibility is irrelevant for @nt{character_literal}s. @end{Ramification} @end{itemize} @end{StaticSem} @begin{RunTime} @PDefn2{Term=[elaboration], Sec=(use_clause)} The elaboration of a @nt{use_clause} has no effect. @end{RunTime} @begin{Examples} @leading@keepnext@i{Example of a use clause in a context clause:} @begin{Example} @key[with] Ada.Calendar; @key[use] Ada; @end{Example} @begin{Wide} @leading@keepnext@i{Example of a use type clause:} @end{Wide} @begin{Example} @key[use type] Rational_Numbers.Rational; --@RI{ see @RefSecNum{Package Specifications and Declarations}} Two_Thirds: Rational_Numbers.Rational := 2/3; @end{Example} @begin{Ramification} In @lquotes@;@key[use] X, Y;@rquotes@;, Y cannot refer to something made visible by the @lquotes@;@key[use]@rquotes@; of X. Thus, it's not (quite) equivalent to @lquotes@;@key[use] X; @key[use] Y;@rquotes@;. If a given declaration is already immediately visible, then a @nt{use_clause} that makes it potentially use-visible has no effect. Therefore, a @nt{use_type_clause} for a type whose declaration appears in a place other than the visible part of a package has no effect; it cannot make a declaration use-visible unless that declaration is already immediately visible. "@key[Use] @key[type] S1;" and "@key[use] @key[type] S2;" are equivalent if S1 and S2 are both subtypes of the same type. In particular, "@key[use] @key[type] S;" and "@key[use] @key[type] S'Base;" are equivalent. @end{Ramification} @begin{Reason} We considered adding a rule that prevented several declarations of views of the same entity that all have the same semantics from cancelling each other out. For example, if a (possibly implicit) @nt{subprogram_declaration} for "+" is potentially use-visible, and a fully conformant renaming of it is also potentially use-visible, then they (annoyingly) cancel each other out; neither one is use-visible. The considered rule would have made just one of them use-visible. We gave up on this idea due to the complexity of the rule. It would have had to account for both overloadable and non-overloadable @nt{renaming_declaration}s, the case where the rule should apply only to some subset of the declarations with the same defining name, and the case of @nt{subtype_declaration}s (since they are claimed to be sufficient for renaming of subtypes). @end{Reason} @end{Examples} @begin{Extend83} The @nt{use_type_clause} is new to Ada 95. @end{Extend83} @begin{DiffWord83} The phrase @lquotes@;omitting from this set any packages that enclose this place@rquotes@; is no longer necessary to avoid making something visible outside its scope, because we explicitly state that the declaration has to be visible in order to be potentially use-visible. @end{DiffWord83} @LabeledClause{Renaming Declarations} @begin{Intro} @redundant[A @nt{renaming_declaration} declares another name for an entity, such as an object, exception, package, subprogram, entry, or generic unit. Alternatively, a @nt{subprogram_renaming_declaration} can be the completion of a previous @nt{subprogram_declaration}.] @end{Intro} @begin{Syntax} @Syn{lhs=,rhs=" @Syn2{object_renaming_declaration} | @Syn2{exception_renaming_declaration} | @Syn2{package_renaming_declaration} | @Syn2{subprogram_renaming_declaration} | @Syn2{generic_renaming_declaration}"} @end{Syntax} @begin{RunTime} @PDefn2{Term=[elaboration], Sec=(renaming_declaration)} The elaboration of a @nt{renaming_declaration} evaluates the @nt{name} that follows the reserved word @key{renames} and thereby determines the view and entity denoted by this name @Defn{renamed view} @Defn{renamed entity} (the @i{renamed view} and @i{renamed entity}). @Redundant[A @nt{name} that denotes the @nt{renaming_declaration} denotes (a new view of) the renamed entity.] @end{RunTime} @begin{Notes} Renaming may be used to resolve name conflicts and to act as a shorthand. Renaming with a different @nt{identifier} or @nt{operator_symbol} does not hide the old @nt{name}; the new @nt{name} and the old @nt{name} need not be visible at the same places. A task or protected object that is declared by an explicit @nt{object_declaration} can be renamed as an object. However, a single task or protected object cannot be renamed since the corresponding type is anonymous (meaning it has no nameable subtypes). For similar reasons, an object of an anonymous array or access type cannot be renamed. @leading@keepnext@;A subtype defined without any additional constraint can be used to achieve the effect of renaming another subtype (including a task or protected subtype) as in @begin{Example} @key[subtype] Mode @key[is] Ada.Text_IO.File_Mode; @end{Example} @end{Notes} @begin{DiffWord83} The second sentence of RM83-8.5(3), @lquotes@;At any point where a renaming declaration is visible, the identifier, or operator symbol of this declaration denotes the renamed entity.@rquotes@; is incorrect. It doesn't say directly visible. Also, such an @nt{identifier} might resolve to something else. The verbiage about renamings being legal @lquotes@;only if exactly one...@rquotes@;, which appears in RM83-8.5(4) (for objects) and RM83-8.5(7) (for subprograms) is removed, because it follows from the normal rules about overload resolution. For language lawyers, these facts are obvious; for programmers, they are irrelevant, since failing these tests is highly unlikely. @end{DiffWord83} @LabeledSubClause{Object Renaming Declarations} @begin{Intro} @redundant[An @nt{object_renaming_declaration} is used to rename an object.] @end{Intro} @begin{Syntax} @Syn{lhs=,rhs="@Syn2{defining_identifier} : @Syn2{subtype_mark} @key{renames} @SynI{object_}@Syn2{name};"} @end{Syntax} @begin{Resolution} The type of the @SynI{object_}@nt{name} shall resolve to the type determined by the @nt{subtype_mark}. @begin{Reason} @leading@;A previous version of Ada 9X used the usual @lquotes@;expected type@rquotes@; wording:@* @lquotes@;The expected type for the @SynI{object_}@nt{name} is that determined by the @nt{subtype_mark}.@rquotes@;@* We changed it so that this would be illegal: @begin{Example} X: T; Y: T'Class @key[renames] X; --@RI{ Illegal!} @end{Example} @leading@;When the above was legal, it was unclear whether Y was of type T or T'Class. Note that we still allow this: @begin{Example} Z: T'Class := ...; W: T @key[renames] F(Z); @end{Example} where F is a function with a controlling parameter and result. This is admittedly a bit odd. Note that the matching rule for generic formal parameters of mode @key[in out] was changed to keep it consistent with the rule for renaming. That makes the rule different for @key[in] vs. @key[in out]. @end{Reason} @end{Resolution} @begin{Legality} The renamed entity shall be an object. @ChgRef{Version=[1],Kind=[Revised],Ref=[8652/0017]} The renamed entity shall not be a subcomponent that depends on discriminants of a variable whose nominal subtype is unconstrained, unless this subtype is indefinite, or the variable is aliased. A @nt{slice} of an array shall not be renamed if this restriction disallows renaming of the array. @Chg{New=[In addition to the places where Legality Rules normally apply, these rules apply also in the private part of an instance of a generic unit. These rules also apply for a renaming that appears in the body of a generic unit, with the additional requirement that even if the nominal subtype of the variable is indefinite, its type shall not be a descendant of an untagged generic formal derived type.],Old=[]} @begin{Reason} This prevents renaming of subcomponents that might disappear, which might leave dangling references. Similar restrictions exist for the Access attribute. @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0017]} @Chg{New=[@Leading@;The "recheck on instantiation" and "assume-the-worst in the body" restrictions on generics are necessary to avoid renaming of components which could disappear even when the nominal subtype would prevent the problem:],Old=[]} @begin{Example} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key{type} T1 (D1 : Boolean) @key{is} @key{record} @key{case} D1 @key{is} @key{when} False => C1 : Integer; @key{when} True => @key{null}; @key{end} @key{case}; @key{end} @key{record};],Old=[]} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key{generic} @key{type} F @key{is} @key{new} T1; X : @key{in out} F; @key{package} G @key{is} C1_Ren : Integer @key{renames} X.C1; @key{end} G;],Old=[]} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[@key{type} T2 (D2 : Boolean := True) @key{is} @key{new} T1 (D1 => D2); @Comment{Blank line} Y : T2; @Comment{Blank line} @key{package} I @key{is new} G (T2, Y); @Comment{Blank line} Y := (D1 => True); -- Oops! What happened to I.C1_Ren?],Old=[]} @end{Example} @end{Reason} @begin{ImplNote} Note that if an implementation chooses to deallocate-then-reallocate on @nt{assignment_@!statement}s assigning to unconstrained definite objects, then it cannot represent renamings and access values as simple addresses, because the above rule does not apply to all components of such an object. @end{ImplNote} @begin{Ramification} If it is a generic formal object, then the assume-the-best or assume-the-worst rules are applied as appropriate. @end{Ramification} @end{Legality} @begin{StaticSem} An @nt{object_renaming_declaration} declares a new view @Redundant{of the renamed object} whose properties are identical to those of the renamed view. @Redundant[Thus, the properties of the renamed object are not affected by the @nt{renaming_declaration}. In particular, its value and whether or not it is a constant are unaffected; similarly, the constraints that apply to an object are not affected by renaming (any constraint implied by the @nt{subtype_mark} of the @nt{object_renaming_declaration} is ignored).] @begin{Discussion} Because the constraints are ignored, it is a good idea to use the nominal subtype of the renamed object when writing an @nt{object_renaming_declaration}. @end{Discussion} @end{StaticSem} @begin{Examples} @leading@keepnext@i{Example of renaming an object:} @begin{Example} @key[declare] L : Person @key[renames] Leftmost_Person; --@RI{ see @RefSecNum{Incomplete Type Declarations}} @key[begin] L.Age := L.Age + 1; @key[end]; @end{Example} @end{Examples} @begin{DiffWord83} The phrase @lquotes@;subtype ... as defined in a corresponding object declaration, component declaration, or component subtype indication,@rquotes@; from RM83-8.5(5), is incorrect in Ada 95; therefore we removed it. It is incorrect in the case of an object with an indefinite unconstrained nominal subtype. @end{DiffWord83} @LabeledSubClause{Exception Renaming Declarations} @begin{Intro} @redundant[An @nt{exception_renaming_declaration} is used to rename an exception.] @end{Intro} @begin{Syntax} @Syn{lhs=,rhs="@Syn2{defining_identifier} : @key{exception} @key{renames} @SynI{exception_}@Syn2{name};"} @end{Syntax} @begin{Legality} The renamed entity shall be an exception. @end{Legality} @begin{StaticSem} An @nt{exception_renaming_declaration} declares a new view @Redundant{of the renamed exception}. @end{StaticSem} @begin{Examples} @leading@keepnext@i{Example of renaming an exception:} @begin{Example} EOF : @key[exception] @key[renames] Ada.IO_Exceptions.End_Error; @RI{-- see @RefSecNum{Exceptions in Input-Output}} @end{Example} @end{Examples} @LabeledSubClause{Package Renaming Declarations} @begin{Intro} @redundant[A @nt{package_renaming_declaration} is used to rename a package.] @end{Intro} @begin{Syntax} @Syn{lhs=,rhs="@key{package} @Syn2{defining_program_unit_name} @key{renames} @SynI{package_}@Syn2{name};"} @end{Syntax} @begin{Legality} The renamed entity shall be a package. @end{Legality} @begin{StaticSem} A @nt{package_renaming_declaration} declares a new view @Redundant{of the renamed package}. @end{StaticSem} @begin{Examples} @leading@keepnext@i{Example of renaming a package:} @begin{Example} @key[package] TM @key[renames] Table_Manager; @end{Example} @end{Examples} @LabeledSubClause{Subprogram Renaming Declarations} @begin{Intro} A @nt{subprogram_renaming_declaration} can serve as the completion of a @nt{subprogram_declaration}; @Defn{renaming-as-body} such a @nt{renaming_declaration} is called a @i{renaming-as-body}. @Defn{renaming-as-declaration} A @nt{subprogram_renaming_declaration} that is not a completion is called a @i{renaming-as-declaration}@Redundant[, and is used to rename a subprogram (possibly an enumeration literal) or an entry]. @begin{Ramification} A renaming-as-body is a declaration, as defined in Section 3. @end{Ramification} @end{Intro} @begin{Syntax} @Syn{lhs=,rhs="@Syn2{subprogram_specification} @key{renames} @SynI{callable_entity_}@Syn2{name};"} @end{Syntax} @begin{Resolution} @PDefn2{Term=[expected profile], Sec=(subprogram_renaming_declaration)} The expected profile for the @i(callable_entity_)@nt is the profile given in the @nt. @end{Resolution} @begin{Legality} The profile of a renaming-as-declaration shall be mode-conformant with that of the renamed callable entity. @Defn2{Term=[mode conformance],Sec=(required)} @ChgRef{Version=[1],Kind=[Revised],Ref=[8652/0027],Ref=[8652/0028]} The profile of a renaming-as-body @Chg{New=[],Old=[shall be subtype-conformant with that of the renamed callable entity, and ]}shall conform fully to that of the declaration it completes. @Defn2{Term=[full conformance],Sec=(required)} If the renaming-as-body completes that declaration before the subprogram it declares is frozen, @Chg{New=[the profile shall be mode-conformant @Defn2{Term=[mode conformance],Sec=(required)}with that of the renamed callable entity and ],Old=[]}the subprogram it declares takes its convention from the renamed subprogram; otherwise@Chg{New=[, the profile shall be subtype-conformant with that of the renamed callable entity and],Old=[]} the convention of the renamed subprogram shall not be Intrinsic. @Defn2{Term=[subtype conformance],Sec=(required)} @Chg{New=[A renaming-as-body is illegal if the declaration occurs before the subprogram whose declaration it completes is frozen, and the renaming renames the subprogram itself, through one or more subprogram renaming declarations, none of whose subprograms has been frozen.],Old=[]} @begin{Reason} @ChgRef{Version=[1],Kind=[Revised]} The @Chg{New=[otherwise part of the second sentence],Old=[first part of the first sentence]} is to allow an implementation of a renaming-as-body as a single jump instruction to the target subprogram. Among other things, this prevents a subprogram from being completed with a renaming of an entry. (In most cases, the target of the jump can be filled in at link time. In some cases, such as a renaming of a name like "A(I).@key[all]", an indirect jump is needed. Note that the name is evaluated at renaming time, not at call time.) @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0028]} @Chg{New=[The first part of the second sentence is intended to allow renaming-as-body of predefined operators before the @nt{subprogram_declaration} is frozen. For some types (such as integer types), the parameter type for operators is the base type, and it would be very strange for@* @f{@ @ @ @key{function} Equal (A, B : @key{in} T) @key{return} Boolean;}@* @f{@ @ @ @key{function} Equal (A, B : @key{in} T) @key{return} Boolean @key{renames} "=";}@* to be illegal. (Note that predefined operators cannot be renamed this way after the @nt{subprogram_declaration} is frozen, as they have convention Intrinsic.)],Old=[]} @ChgRef{Version=[1],Kind=[Revised]} The @Chg{New=[],Old=[second part of the ]}first sentence is the normal rule for completions of @nt{subprogram_declaration}s. @end{Reason} @begin{Ramification} An @nt{entry_declaration}, unlike a @nt{subprogram_declaration}, cannot be completed with a @nt{renaming_@!declaration}. Nor can a @nt{generic_@!subprogram_@!declaration}. The syntax rules prevent a protected subprogram declaration from being completed by a renaming. This is fortunate, because it allows us to avoid worrying about whether the implicit protected object parameter of a protected operation is involved in the conformance rules. @end{Ramification} @begin{Reason} @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0027]} @Chg{New=[Circular renames before freezing is illegal, as the compiler would not be able to determine the convention of the subprogram. Other circular renames are handled below, see @BoundedTitle.],Old=[]} @end{Reason} A @nt{name} that denotes a formal parameter of the @nt{subprogram_specification} is not allowed within the @i{callable_entity_}@nt{name}. @begin{Reason} @leading@keepnext@;This is to prevent things like this: @begin{Example} @key[function] F(X : Integer) @key[return] Integer @key[renames] Table(X).@key[all]; @end{Example} @begin{Wide} @leading@;A similar rule in @RefSecNum{Subprogram Declarations} forbids things like this: @end{Wide} @begin{Example} @key[function] F(X : Integer; Y : Integer := X) @key[return] Integer; @end{Example} @end{Reason} @end{Legality} @begin{StaticSem} A renaming-as-declaration declares a new view of the renamed entity. The profile of this new view takes its subtypes, parameter modes, and calling convention from the original profile of the callable entity, while taking the formal parameter @nt{name}s and @nt{default_expression}s from the profile given in the @nt{subprogram_renaming_declaration}. The new view is a function or procedure, never an entry. @begin{Honest} When renaming an entry as a procedure, the compile-time rules apply as if the new view is a procedure, but the run-time semantics of a call are that of an entry call. @end{Honest} @begin{Ramification} For example, it is illegal for the @nt{entry_call_statement} of a @nt{timed_entry_call} to call the new view. But what looks like a procedure call will do things like barrier waiting. @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0105]} @Chg{New=[All properties of the renamed entity are inherited by the new view. In particular, if the renamed entity is abstract or requires overridding (see @RefSecNum{Abstract Types and Subprograms}), the new view also is abstract or requires overridding. (The renaming will often be illegal in these cases, as a renaming cannot be overridden.)],Old=[]} @end{Ramification} @end{StaticSem} @begin{RunTime} @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0014]} @Chg{New=[For a call to a subprogram whose body is given as a renaming-as-body, the execution of the renaming-as-body is equivalent to the execution of a @nt{subprogram_body} that simply calls the renamed subprogram with its formal parameters as the actual parameters and, if it is a function, returns the value of the call.],Old=[]} @begin{Ramification} @ChgRef{Version=[1],Kind=[Added]} @Chg{New=[This implies that the subprogram completed by the renames-as-body has its own elaboration check.],Old=[]} @end{Ramification} For a call on a renaming of a dispatching subprogram that is overridden, if the overriding occurred before the renaming, then the body executed is that of the overriding declaration, even if the overriding declaration is not visible at the place of the renaming; otherwise, the inherited or predefined subprogram is called. @begin{Discussion} Note that whether or not the renaming is itself primitive has nothing to do with the renamed subprogram. Note that the above rule is only for tagged types. @leading@keepnext@;Consider the following example: @begin{Example} @key[package] P @key[is] @key[type] T @key[is] @key[tagged] @key[null] @key[record]; @key[function] Predefined_Equal(X, Y : T) @key[return] Boolean @key[renames] "="; @key[private] @key[function] "="(X, Y : T) @key[return] Boolean; --@RI{ Override predefined "=".} @key[end] P; @key[with] P; @key[use] P; @key[package] Q @key[is] @key[function] User_Defined_Equal(X, Y : T) @key[return] Boolean @key[renames] P."="; @key[end] Q; @end{Example} A call on Predefined_Equal will execute the predefined equality operator of T, whereas a call on User_Defined_Equal will execute the body of the overriding declaration in the private part of P. Thus a renaming allows one to squirrel away a copy of an inherited or predefined subprogram before later overriding it. @Defn2{Term=[squirrel away],Sec=(included in fairness to alligators)} @end{Discussion} @end{RunTime} @begin{Bounded} @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0027]} @Chg{New=[@Defn2{Term=[Program_Error],Sec=(raised by failure of run-time check)} @Defn2{Term=[Storage_Error],Sec=(raised by failure of run-time check)} If a subprogram directly or indirectly renames itself, then it is a bounded error to call that subprogram. Possible consequences are that Program_Error or Storage_Error is raised, or that the call results in infinite recursion.],Old=[]} @begin{Reason} @ChgRef{Version=[1],Kind=[Added],Ref=[8652/0027]} @Chg{New=[This has to be a bounded error, as it is possible for a renames-as-body appearing in a package body to cause this problem. Thus it is not possible in general to detect this problem at compile-time.],Old=[]} @end{Reason} @end{Bounded} @begin{Notes} A procedure can only be renamed as a procedure. A function whose @nt{defining_designator} is either an @nt{identifier} or an @nt{operator_symbol} can be renamed with either an @nt{identifier} or an @nt{operator_symbol}; for renaming as an operator, the subprogram specification given in the @nt{renaming_declaration} is subject to the rules given in @RefSecNum{Overloading of Operators} for operator declarations. Enumeration literals can be renamed as functions; similarly, @nt{attribute_reference}s that denote functions (such as references to Succ and Pred) can be renamed as functions. An entry can only be renamed as a procedure; the new @nt{name} is only allowed to appear in contexts that allow a procedure @nt{name}. An entry of a family can be renamed, but an entry family cannot be renamed as a whole. The operators of the root numeric types cannot be renamed because the types in the profile are anonymous, so the corresponding specifications cannot be written; the same holds for certain attributes, such as Pos. Calls with the new @nt{name} of a renamed entry are @nt{procedure_call_statement}s and are not allowed at places where the syntax requires an @nt{entry_call_statement} in @nt{conditional_} and @nt{timed_entry_call}s, nor in an @nt{asynchronous_select}; similarly, the Count attribute is not available for the new @nt{name}. The primitiveness of a renaming-as-declaration is determined by its profile, and by where it occurs, as for any declaration of (a view of) a subprogram; primitiveness is not determined by the renamed view. In order to perform a dispatching call, the subprogram name has to denote a primitive subprogram, not a non-primitive renaming of a primitive subprogram. @begin{Reason} A @nt{subprogram_renaming_declaration} could more properly be called @nt{renaming_@!as_@!subprogram_@!declaration}, since you're renaming something as a subprogram, but you're not necessarily renaming a subprogram. But that's too much of a mouthful. Or, alternatively, we could call it a @nt{callable_@!entity_@!renaming_@!declaration}, but that's even worse. Not only is it a mouthful, it emphasizes the entity being renamed, rather than the new view, which we think is a bad idea. We'll live with the oddity. @end{Reason} @end{Notes} @begin{Examples} @leading@keepnext@i{Examples of subprogram renaming declarations:} @begin{Example} @key[procedure] My_Write(C : @key[in] Character) @key[renames] Pool(K).Write; --@RI{ see @RefSecNum{Selected Components}} @key[function] Real_Plus(Left, Right : Real ) @key[return] Real @key[renames] "+"; @key[function] Int_Plus (Left, Right : Integer) @key[return] Integer @key[renames] "+"; @key[function] Rouge @key[return] Color @key[renames] Red; --@RI{ see @RefSecNum{Enumeration Types}} @key[function] Rot @key[return] Color @key[renames] Red; @key[function] Rosso @key[return] Color @key[renames] Rouge; @key[function] Next(X : Color) @key[return] Color @key[renames] Color'Succ; --@RI{ see @RefSecNum{Enumeration Types}} @end{Example} @begin{Wide} @leading@keepnext@i{Example of a subprogram renaming declaration with new parameter names:} @end{Wide} @begin{Example} @key[function] "*" (X,Y : Vector) @key[return] Real @key[renames] Dot_Product; --@RI{ see @RefSecNum{Subprogram Declarations}} @end{Example} @begin{Wide} @leading@keepnext@i{Example of a subprogram renaming declaration with a new default expression:} @end{Wide} @begin{Example} @key[function] Minimum(L : Link := Head) @key[return] Cell @key[renames] Min_Cell; --@RI{ see @RefSecNum{Subprogram Declarations}} @end{Example} @end{Examples} @LabeledSubClause{Generic Renaming Declarations} @begin{Intro} @redundant[A @nt{generic_renaming_declaration} is used to rename a generic unit.] @end{Intro} @begin{Syntax} @Syn{tabs=[P22], lhs=,rhs=" @key{generic package}@\@Syn2{defining_program_unit_name} @key{renames} @SynI{generic_package_}@Syn2{name}; | @key{generic procedure}@\@Syn2{defining_program_unit_name} @key{renames} @SynI{generic_procedure_}@Syn2{name}; | @key{generic function}@\@Syn2{defining_program_unit_name} @key{renames} @SynI{generic_function_}@Syn2{name};"} @end{Syntax} @begin{Legality} The renamed entity shall be a generic unit of the corresponding kind. @end{Legality} @begin{StaticSem} A @nt{generic_renaming_declaration} declares a new view @Redundant{of the renamed generic unit}. @end{StaticSem} @begin{Notes} Although the properties of the new view are the same as those of the renamed view, the place where the @nt occurs may affect the legality of subsequent renamings and instantiations that denote the @nt, in particular if the renamed generic unit is a library unit (see @RefSecNum{Compilation Units - Library Units}). @end{Notes} @begin{Examples} @leading@keepnext@i{Example of renaming a generic unit:} @begin{Example} @key[generic package] Enum_IO @key[renames] Ada.Text_IO.Enumeration_IO; @RI{-- see @RefSecNum{Input-Output for Enumeration Types}} @end{Example} @end{Examples} @begin{Extend83} Renaming of generic units is new to Ada 95. It is particularly important for renaming child library units that are generic units. For example, it might be used to rename Numerics.Generic_Elementary_Functions as simply Generic_Elementary_Functions, to match the name for the corresponding Ada-83-based package. @end{Extend83} @begin{DiffWord83} The information in RM83-8.6, @lquotes@;The Package Standard,@rquotes@; has been updated for the child unit feature, and moved to @RefSecNum{Predefined Language Environment}, except for the definition of @lquotes@;predefined type,@rquotes@; which has been moved to @RefSecNum{Type Declarations}. @end{DiffWord83} @LabeledClause{The Context of Overload Resolution} @begin{Intro} @redundant[@Defn{overload resolution} Because declarations can be overloaded, it is possible for an occurrence of a usage name to have more than one possible interpretation; in most cases, ambiguity is disallowed. This clause describes how the possible interpretations resolve to the actual interpretation. @Defn{overloading rules} Certain rules of the language (the @ResolutionTitle) are considered @lquotes@;overloading rules@rquotes@;. If a possible interpretation violates an overloading rule, it is assumed not to be the intended interpretation; some other possible interpretation is assumed to be the actual interpretation. On the other hand, violations of non-overloading rules do not affect which interpretation is chosen; instead, they cause the construct to be illegal. To be legal, there usually has to be exactly one acceptable interpretation of a construct that is a @lquotes@;complete context@rquotes@;, not counting any nested complete contexts. @Defn2{Term=[grammar],Sec=(resolution of ambiguity)} The syntax rules of the language and the visibility rules given in @RefSecNum{Visibility} determine the possible interpretations. Most type checking rules (rules that require a particular type, or a particular class of types, for example) are overloading rules. Various rules for the matching of formal and actual parameters are overloading rules.] @end{Intro} @begin{MetaRules} The type resolution rules are intended to minimize the need for implicit declarations and preference rules associated with implicit conversion and dispatching operations. @end{MetaRules} @begin{Resolution} @leading@Defn{complete context} @Redundant{Overload resolution is applied separately to each @i{complete context}, not counting inner complete contexts.} Each of the following constructs is a @i{complete context}: @begin{itemize} A @nt{context_item}. A @nt{declarative_item} or declaration. @begin{Ramification} A @nt{loop_parameter_specification} is a declaration, and hence a complete context. @end{Ramification} A @nt{statement}. A @nt{pragma_argument_association}. @begin{Reason} We would make it the whole @nt{pragma}, except that certain pragma arguments are allowed to be ambiguous, and ambiguity applies to a complete context. @end{Reason} The @nt{expression} of a @nt{case_statement}. @begin{Ramification} This means that the @nt{expression} is resolved without looking at the choices. @end{Ramification} @end{itemize} @leading@Defn2{Term=[interpretation], Sec=(of a complete context)} @Defn2{Term=[overall interpretation], Sec=(of a complete context)} An (overall) @i{interpretation} of a complete context embodies its meaning, and includes the following information about the constituents of the complete context, not including constituents of inner complete contexts: @begin{Itemize} for each constituent of the complete context, to which syntactic categories it belongs, and by which syntax rules; and @begin{Ramification} Syntactic categor@i{ies} is plural here, because there are lots of trivial productions @em an @nt{expression} might also be all of the following, in this order: @nt{identifier}, @nt{name}, @nt{primary}, @nt{factor}, @nt{term}, @nt{simple_expression}, and @nt{relation}. Basically, we're trying to capture all the information in the parse tree here, without using compiler-writer's jargon like @lquotes@;parse tree@rquotes@;. @end{Ramification} for each usage name, which declaration it denotes (and, therefore, which view and which entity it denotes); and @begin{Ramification} In most cases, a usage name denotes the view declared by the denoted declaration. However, in certain cases, a usage name that denotes a declaration and appears inside the declarative region of that same declaration, denotes the current instance of the declaration. For example, within a @nt{task_body}, a usage name that denotes the @nt{task_type_declaration} denotes the object containing the currently executing task, and not the task type declared by the declaration. @end{Ramification} for a complete context that is a @nt{declarative_item}, whether or not it is a completion of a declaration, and (if so) which declaration it completes. @end{Itemize} @begin{Ramification} Unfortunately, we are not confident that the above list is complete. We'll have to live with that. @end{Ramification} @begin{Honest} For @lquotes@;possible@rquotes@; interpretations, the above information is tentative. @end{Honest} @begin{Discussion} A possible interpretation (an @i{input} to overload resolution) contains information about what a usage name @i{might} denote, but what it actually @i{does} denote requires overload resolution to determine. Hence the term @lquotes@;tentative@rquotes@; is needed for possible interpretations; otherwise, the definition would be circular. @end{Discussion} @Defn{possible interpretation} A @i{possible interpretation} is one that obeys the syntax rules and the visibility rules. @Defn{acceptable interpretation} @Defn2{Term=[resolve],Sec=(overload resolution)} @Defn2{Term=[interpretation],Sec=(overload resolution)} An @i{acceptable interpretation} is a possible interpretation that obeys the @i{overloading rules}@Redundant{, that is, those rules that specify an expected type or expected profile, or specify how a construct shall @i(resolve) or be @i(interpreted).} @begin{Honest} One rule that falls into this category, but does not use the above-mentioned magic words, is the rule about numbers of parameter associations in a call (see @RefSecNum{Subprogram Calls}). @end{Honest} @begin{Ramification} The @ResolutionName@;s are the ones that appear under the @ResolutionTitle heading. Some @SyntaxName@;s are written in English, instead of BNF. No rule is a @SyntaxName or @ResolutionName unless it appears under the appropriate heading. @end{Ramification} @Defn2{Term=[interpretation], Sec=(of a constituent of a complete context)} The @i{interpretation} of a constituent of a complete context is determined from the overall interpretation of the complete context as a whole. @Redundant{Thus, for example, @lquotes@;interpreted as a @nt{function_call},@rquotes@; means that the construct's interpretation says that it belongs to the syntactic category @nt{function_call}.} @leading@Defn{denote} @Redundant[Each occurrence of] a usage name @i{denotes} the declaration determined by its interpretation. It also denotes the view declared by its denoted declaration, except in the following cases: @begin{Ramification} As explained below, a pragma argument is allowed to be ambiguous, so it can denote several declarations, and all of the views declared by those declarations. @end{Ramification} @begin(itemize) @Defn2{Term=[current instance], Sec=(of a type)} If a usage name appears within the declarative region of a @nt{type_declaration} and denotes that same @nt{type_declaration}, then it denotes the @i{current instance} of the type (rather than the type itself). The current instance of a type is the object or value of the type that is associated with the execution that evaluates the usage name. @begin{Reason} This is needed, for example, for references to the Access attribute from within the @nt{type_declaration}. Also, within a @nt{task_body} or @nt{protected_body}, we need to be able to denote the current task or protected object. (For a @nt{single_task_declaration} or @nt{single_protected_declaration}, the rule about current instances is not needed.) @end{Reason} @Defn2{Term=[current instance], Sec=(of a generic unit)} If a usage name appears within the declarative region of a @nt{generic_declaration} (but not within its @nt{generic_formal_part}) and it denotes that same @nt{generic_declaration}, then it denotes the @i{current instance} of the generic unit (rather than the generic unit itself). See also @RefSecNum{Generic Instantiation}. @begin{Honest} The current instance of a generic unit is the instance created by whichever @nt{generic_instantiation} is of interest at any given time. @end{Honest} @begin{Ramification} Within a @nt{generic_formal_part}, a @nt{name} that denotes the @nt{generic_declaration} denotes the generic unit, which implies that it is not overloadable. @end{Ramification} @end(itemize) A usage name that denotes a view also denotes the entity of that view. @begin{Ramification} Usually, a usage name denotes only one declaration, and therefore one view and one entity. @end{Ramification} @leading@RootDefn[expected type] The @i(expected type) for a given @nt, @nt, or other construct determines, according to the @i{type resolution rules} given below, the types considered for the construct during overload resolution. @Defn{type resolution rules} @Redundant[ The type resolution rules provide support for class-wide programming, universal numeric literals, dispatching operations, and anonymous access types:] @begin{Ramification} Expected types are defined throughout the RM95. The most important definition is that, for a subprogram, the expected type for the actual parameter is the type of the formal parameter. The type resolution rules are trivial unless either the actual or expected type is universal, class-wide, or of an anonymous access type. @end{Ramification} @begin{Itemize} @PDefn2{Term=[type resolution rules], Sec=(if any type in a specified class of types is expected)} @PDefn2{Term=[type resolution rules], Sec=(if expected type is universal or class-wide)} If a construct is expected to be of any type in a class of types, or of the universal or class-wide type for a class, then the type of the construct shall resolve to a type in that class or to a universal type that covers the class. @begin{Ramification} This matching rule handles (among other things) cases like the Val attribute, which denotes a function that takes a parameter of type @i(universal_integer). The last part of the rule, @lquotes@;or to a universal type that includes the class@rquotes@; implies that if the expected type for an expression is @i{universal_fixed}, then an expression whose type is @i{universal_real} (such as a real literal) is OK. @end{Ramification} @leading@PDefn2{Term=[type resolution rules], Sec=(if expected type is specific)} If the expected type for a construct is a specific type @i(T), then the type of the construct shall resolve either to @i(T), or: @begin{Ramification} @PDefn{Beaujolais effect} This rule is @i{not} intended to create a preference for the specific type @em such a preference would cause Beaujolais effects. @end{Ramification} @begin(itemize) to @i(T)'Class; or @begin{Ramification} This will only be legal as part of a call on a dispatching operation; see @RefSec(Dispatching Operations of Tagged Types). Note that that rule is not a @ResolutionName. @end{Ramification} to a universal type that covers @i(T); or when @i(T) is an anonymous access type (see @RefSecNum{Access Types}) with designated type @i(D), to an access-to-variable type whose designated type is @i(D)'Class or is covered by @i(D). @begin{Ramification} Because it says @lquotes@;access-to-variable@rquotes@; instead of @lquotes@;access-to-object,@rquotes@; two subprograms that differ only in that one has a parameter of an access-to-constant type, and the other has an access parameter, are distinguishable during overload resolution. The case where the actual is access-to-@i(D)'Class will only be legal as part of a call on a dispatching operation; see @RefSec(Dispatching Operations of Tagged Types). Note that that rule is not a @ResolutionName. @end{Ramification} @end(itemize) @end{Itemize} @RootDefn[expected profile] In certain contexts, @Redundant[such as in a @nt{subprogram_renaming_declaration},] the @ResolutionTitle define an @i(expected profile) for a given @nt; @Defn2{Term=[profile resolution rule], Sec=(@nt with a given expected profile)} in such cases, the @nt{name} shall resolve to the name of a callable entity whose profile is type conformant with the expected profile. @Defn2{Term=[type conformance],Sec=(required)} @begin{Ramification} The parameter and result @i{sub}types are not used in overload resolution. Only type conformance of profiles is considered during overload resolution. Legality rules generally require at least mode-conformance in addition, but those rules are not used in overload resolution. @end{Ramification} @end{Resolution} @begin{Legality} @Defn2{Term=[single], Sec=[class expected type]} When the expected type for a construct is required to be a @i(single) type in a given class, the type expected for the construct shall be determinable solely from the context in which the construct appears, excluding the construct itself, but using the requirement that it be in the given class; the type of the construct is then this single expected type. Furthermore, the context shall not be one that expects any type in some class that contains types of the given class; in particular, the construct shall not be the operand of a @nt{type_conversion}. @begin{Ramification} For example, the expected type for the literal @key{null} is required to be a single access type. But the expected type for the operand of a @nt{type_conversion} is any type. Therefore, the literal @key{null} is not allowed as the operand of a @nt{type_conversion}. This is true even if there is only one access type in scope. The reason for these rules is so that the compiler will not have to search @lquotes@;everywhere@rquotes@; to see if there is exactly one type in a class in scope. @end{Ramification} A complete context shall have at least one acceptable interpretation; if there is exactly one, then that one is chosen. @begin{Ramification} This, and the rule below about ambiguity, are the ones that suck in all the @SyntaxName@;s and @ResolutionName@;s as compile-time rules. Note that this and the ambiguity rule have to be @LegalityName@;s. @end{Ramification} @Defn2{Term=[preference], Sec=(for root numeric operators and @nts)} There is a @i{preference} for the primitive operators (and @nts) of the root numeric types @i{root_integer} and @i{root_real}. In particular, if two acceptable interpretations of a constituent of a complete context differ only in that one is for a primitive operator (or @nt) of the type @i{root_integer} or @i{root_real}, and the other is not, the interpretation using the primitive operator (or @nt) of the root numeric type is @i{preferred}. @begin{Reason} @leading@;The reason for this preference is so that expressions involving literals and named numbers can be unambiguous. For example, without the preference rule, the following would be ambiguous: @begin{Example} N : @key[constant] := 123; @key[if] N > 100 @key[then] --@RI{ Preference for root_integer "<" operator.} ... @key[end] @key[if]; @end{Example} @end{Reason} For a complete context, if there is exactly one overall acceptable interpretation where each constituent's interpretation is the same as or preferred (in the above sense) over those in all other overall acceptable interpretations, then that one overall acceptable interpretation is chosen. @Defn{ambiguous} Otherwise, the complete context is @i{ambiguous}. A complete context other than a @nt{pragma_argument_association} shall not be ambiguous. A complete context that is a @nt{pragma_argument_association} is allowed to be ambiguous (unless otherwise specified for the particular pragma), but only if every acceptable interpretation of the pragma argument is as a @nt{name} that statically denotes a callable entity. @PDefn2{Term=[denote],Sec=(name used as a pragma argument)} Such a @nt{name} denotes all of the declarations determined by its interpretations, and all of the views declared by these declarations. @begin{Ramification} This applies to Inline, Suppress, Import, Export, and Convention @nt{pragma}s. For example, it is OK to say @lquotes@;@key[pragma] Suppress(Elaboration_Check, On => P.Q);@rquotes@;, even if there are two directly visible P's, and there are two Q's declared in the visible part of each P. In this case, P.Q denotes four different declarations. This rule also applies to certain pragmas defined in the Specialized Needs Annexes. It almost applies to Pure, Elaborate_Body, and Elaborate_All @nt{pragma}s, but those can't have overloading for other reasons. Note that if a pragma argument denotes a @i{call} to a callable entity, rather than the entity itself, this exception does not apply, and ambiguity is disallowed. Note that we need to carefully define which pragma-related rules are @ResolutionName@;s, so that, for example, a @nt{pragma} Inline does not pick up subprograms declared in enclosing declarative regions, and therefore make itself illegal. We say @lquotes@;statically denotes@rquotes@; in the above rule in order to avoid having to worry about how many times the @nt{name} is evaluated, in case it denotes more than one callable entity. @end{Ramification} @end{Legality} @begin{Notes} If a usage name has only one acceptable interpretation, then it denotes the corresponding entity. However, this does not mean that the usage name is necessarily legal since other requirements exist which are not considered for overload resolution; for example, the fact that an expression is static, whether an object is constant, mode and subtype conformance rules, freezing rules, order of elaboration, and so on. @NoPrefix@;Similarly, subtypes are not considered for overload resolution (the violation of a constraint does not make a program illegal but raises an exception during program execution). @end{Notes} @begin{Incompatible83} @PDefn{Beaujolais effect} The new preference rule for operators of root numeric types is upward incompatible, but only in cases that involved @i(Beaujolais) effects in Ada 83. Such cases are ambiguous in Ada 95. @end{Incompatible83} @begin{Extend83} The rule that allows an expected type to match an actual expression of a universal type, in combination with the new preference rule for operators of root numeric types, subsumes the Ada 83 "implicit conversion" rules for universal types. @end{Extend83} @begin{DiffWord83} In Ada 83, it is not clear what the @lquotes@;syntax rules@rquotes@; are. AI83-00157 states that a certain textual rule is a syntax rule, but it's still not clear how one tells in general which textual rules are syntax rules. We have solved the problem by stating exactly which rules are syntax rules @em the ones that appear under the @lquotes@;@SyntaxTitle@rquotes@; heading. RM83 has a long list of the @lquotes@;forms@rquotes@; of rules that are to be used in overload resolution (in addition to the syntax rules). It is not clear exactly which rules fall under each form. We have solved the problem by explicitly marking all rules that are used in overload resolution. Thus, the list of kinds of rules is unnecessary. It is replaced with some introductory (intentionally vague) text explaining the basic idea of what sorts of rules are overloading rules. It is not clear from RM83 what information is embodied in a @lquotes@;meaning@rquotes@; or an @lquotes@;interpretation.@rquotes@; @lquotes@;Meaning@rquotes@; and @lquotes@;interpretation@rquotes@; were intended to be synonymous; we now use the latter only in defining the rules about overload resolution. @lquotes@;Meaning@rquotes@; is used only informally. This clause attempts to clarify what is meant by @lquotes@;interpretation.@rquotes@; @NoPrefix@;For example, RM83 does not make it clear that overload resolution is required in order to match @nt{subprogram_bodies} with their corresponding declarations (and even to tell whether a given @nt{subprogram_body} is the completion of a previous declaration). Clearly, the information needed to do this is part of the @lquotes@;interpretation@rquotes@; of a @nt{subprogram_body}. The resolution of such things is defined in terms of the @lquotes@;expected profile@rquotes@; concept. Ada 95 has some new cases where expected profiles are needed @em the resolution of P'Access, where P might denote a subprogram, is an example. @Leading@NoPrefix@;RM83-8.7(2) might seem to imply that an interpretation embodies information about what is denoted by each usage name, but not information about which syntactic category each construct belongs to. However, it seems necessary to include such information, since the Ada grammar is highly ambiguous. For example, X(Y) might be a @nt{function_call} or an @nt{indexed_component}, and no context-free/syntactic information can tell the difference. It seems like we should view X(Y) as being, for example, @lquotes@;interpreted as a @nt{function_call}@rquotes@; (if that's what overload resolution decides it is). Note that there are examples where the denotation of each usage name does not imply the syntactic category. However, even if that were not true, it seems that intuitively, the interpretation includes that information. Here's an example: @begin{Example} @key[type] T; @key[type] A @key[is] @key[access] T; @key[type] T @key[is] @key[array](Integer @key[range] 1..10) @key[of] A; I : Integer := 3; @key[function] F(X : Integer := 7) @key[return] A; Y : A := F(I); --@RI{ Ambiguous? (We hope so.)} @end{Example} @ChgRef{Version=[1],Kind=[Revised]}@ChgNote{To be consistent with 8652/0006} @NoPrefix@;Consider the declaration of Y (a complete context). In the above example, overload resolution can easily determine the declaration, and therefore the entity, denoted by Y, A, F, and I. However, given all of that information, we still don't know whether F(I) is a @nt{function_call} or an @nt{indexed_component} whose @Chg{New=[@nt{prefix}],Old=[prefix]} is a @nt{function_call}. (In the latter case, it is equivalent to F(7).@key[all](I).) @NoPrefix@;It seems clear that the declaration of Y ought to be considered ambiguous. We describe that by saying that there are two interpretations, one as a @nt{function_call}, and one as an @nt{indexed_component}. These interpretations are both acceptable to the overloading rules. Therefore, the complete context is ambiguous, and therefore illegal. @PDefn{Beaujolais effect} It is the intent that the Ada 95 preference rule for root numeric operators is more locally enforceable than that of RM83-4.6(15). It should also eliminate interpretation shifts due to the addition or removal of a @nt{use_clause} (the so called @i{Beaujolais} effect). RM83-8.7 seems to be missing some complete contexts, such as @nt{pragma_argument_association}s, @nt{declarative_item}s that are not declarations or @nt{representation_clause}s, and @nt{context_item}s. We have added these, and also replaced the @lquotes@;must be determinable@rquotes@; wording of RM83-5.4(3) with the notion that the expression of a @nt{case_statement} is a complete context. Cases like the Val attribute are now handled using the normal type resolution rules, instead of having special cases that explicitly allow things like @lquotes@;any integer type.@rquotes@; @end{DiffWord83}