CVS difference for ai22s/ai22-0029-1.html
--- ai22s/ai22-0029-1.html 2022/10/29 04:38:05 1.3
+++ ai22s/ai22-0029-1.html 2022/11/17 06:38:54 1.4
@@ -1,7 +1,7 @@
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+<html><head><meta content="text/html; charset=UTF-8" http-equiv="content-type"><title>AI22-0029-1/06</title>
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-</head><body class="c16 doc-content"><p class="a3">AI22-0029-1</p>
+</head><body class="c18 doc-content"><p class="a2">AI22-0029-1</p>
<p class="head">!standard 13.3(75.1/3)
- 22-10-24 AI22-0029-1/05</p>
+ 22-11-10 AI22-0029-1/06</p>
<p class="head">!standard 13.3(76)</p>
<p class="head">!class binding interpretation 22-01-24</p>
-<p class="head">!status Corrigendum 1-2022 22-06-23</p>
+<p class="head">!status Corrigendum 1-2022 22-11-10</p>
+<p class="head">!status ARG Approved 13-0-1 22-11-10</p>
<p class="head">!status work item 22-10-18</p>
<p class="head">!status ARG Approved 14-0-0 22-06-23</p>
<p class="head">!status work item 22-01-24</p>
@@ -70,8 +75,8 @@
<span class="ntrm">attribute_definition_clause</span>.</p>
<p class="inst">with:</p>
<p class="word">If two tagged types X and Y both exist in a given partition, X and Y are declared
-by different declarations, and X’External_Tag is the same as Y’External_Tag,
-Program_Error is raised at one or more of the following places:</p>
+by distinct declarations, and a user-specified X’External_Tag is the same as
+Y’External_Tag, then Program_Error is raised at one or more of the following places:</p>
<ul class="wbl"><li>an <span class="ntrm">attribute_definition_clause</span> for
X’External_Tag;</li>
<li>an <span class="ntrm">attribute_definition_clause</span> for Y’External_Tag;</li>
@@ -80,7 +85,9 @@
</ul>
<p class="wind">AARM Reason: The intent is to allow two different implementations here: a fully
static implementation with bind-time construction of an external tag table, and a runtime scheme
-where the elaboration of a type registers the external tag with a manager in the runtime.</p>
+where the elaboration of a type registers the external tag with a manager in the runtime. </p>
+<p class="wind">AARM Discussion: Each instance of a generic unit is considered a distinct
+declaration, as are declarations occurring within distinct instances of the generic unit.</p>
<p class="inst">Replace AARM 13.3(75.e/3) with: [still as part of the Ramification]</p>
<p class="word">This check is on a *pair* of types, and the exception can be raised at any of the
prescribed points. Furthermore, there is a permission (below) that permits the exception to be
@@ -137,42 +144,60 @@
<p class="txts">----</p>
-<p class="txts">It is important that Program_Error never be raised for tagged types that have an
+<p class="text">It is important that Program_Error never be raised for tagged types that have an
External_Tag value chosen by default. Such types may very well never have any uses of an
External_Tag, and it would be very unfriendly for an implementation to terminate the program for a
problem with a feature that is not being used. This implies that it is important for
implementations that use a registration scheme and that make the check when a tagged type is
registered to be able to turn the External_Tag check off for tags that come from different
elaborations of the same declaration.</p>
-
-<p class="txts">We considered making the check depend instead on whether one of the External_Tags
-are user-specified. (If neither are user-specified, then no check would be made, since there would
-only be a problem for a buggy implementation). The Implementation Permission is already worded that
-way (so there is only a permission to disallow a partition when there is at least one user-defined
-External_Tag; a partition that never uses External_Tags can never be rejected).</p>
-
-<p class="txts">This would mean that multiple elaborations of a type declaration with a
-user-specified External_Tag would necessarily raise Program_Error. This would make some sense,
-since one can assume that if someone is specifying the value of an External_Tag, that is being done
-in order that streaming is certain to be compatible with another related program. Having types that
-are ambiguous in such streaming is not likely to work.</p>
-
-<p class="txts">The implementation of such a dynamic check would seem to be easier as well, since
-it would only require one additional bit passed to a tagged type manager to determine whether or
-not the tag comes from a user-specified External_Tag. Keeping track of which declaration a tagged
-type originated from would clearly take a more complex definition.</p>
-
-<p class="text">We didn’t define the check this way for two reasons:</p>
-<ul class="bull"><li>The existing wording already had a different declaration exception. For fixing
-an omission, it seems valuable to avoid making unnecessary changes (even if they seem like an
-improvement);</li>
-<li>For an implementation that has a split tag with static and dynamic parts, the static part is
-going to be associated with exactly one declaration, and thus it can be used to identify the
-declaration with no added data passed to a tagged type manager. It’s likely that some
-implementations are already using this property to implement the existing check. Changing the rule
-would make even more work for those implementations, while the intent of this AI is to make it
-easier to make this check and to allow additional places for it to be made.</li>
-</ul>
+<h2 class="head">!corrigendum 13.3(75.1/5)</h2>
+<p class="text">@drepl</p>
+<p class="text">If a user-specified external tag S'External_Tag is the same as
+T'External_Tag for some other tagged type declared by a different declaration in the partition,
+Program_Error is raised by the elaboration of the @fa{attribute_definition_clause}.</p>
+<p class="text">@dby</p>
+<p class="a14">If two tagged types X and Y both exist in a given partition, X and Y are declared by
+distinct declarations, and a user-specified X’External_Tag is the same as
+Y’External_Tag, then Program_Error is raised at one or more of the following places:</p>
+<p class="a15">@xbullet{an
+@fa{<span class="a19">attribute_definition_clause}</span><span class="a18"> for
+X’External_Tag;}</span></p>
+<p class="a15">@xbullet{an
+@fa{<span class="a19">attribute_definition_clause}</span><span class="a18"> for
+Y’External_Tag;}</span></p>
+<p class="a16">@xbullet{the freezing point of X;}</p>
+<p class="a16">@xbullet{the freezing point of Y.}</p>
+<h2 class="head">!corrigendum 13.3(76/5)</h2>
+<p class="text">@drepl</p>
+<p class="text">In an implementation, the default external tag for each specific tagged type
+declared in a partition shall be distinct, so long as the type is declared outside an instance of a
+generic body. If the compilation unit in which a given tagged type is declared, and all compilation
+units on which it semantically depends, are the same in two different partitions, then the external
+tag for the type shall be the same in the two partitions. What it means for a compilation unit to
+be the same in two different partitions is implementation defined. At a minimum, if the compilation
+unit is not recompiled between building the two different partitions that include it, the
+compilation unit is considered the same in the two partitions.</p>
+<p class="text">@dby</p>
+<p class="text">In an implementation, the default external tag for each specific tagged type
+declared in a partition shall be distinct, so long as the type is declared by a distinct
+declaration and is outside an instance of a generic body. If the compilation unit in which a given
+tagged type is declared, and all compilation units on which it semantically depends, are the same
+in two different partitions, then the external tag for the type shall be the same in the two
+partitions. What it means for a compilation unit to be the same in two different partitions is
+implementation defined. At a minimum, if the compilation unit is not recompiled between building
+the two different partitions that include it, the compilation unit is considered the same in the
+two partitions.</p>
+<h2 class="head">!corrigendum 13.3(76.1/3)</h2>
+<p class="text">@drepl</p>
+<p class="text">If a user-specified external tag S'External_Tag is the same as
+T'External_Tag for some other tagged type declared by a different declaration in the partition,
+the partition may be rejected.</p>
+<p class="text">@dby</p>
+<p class="text">If a user-specified external tag X'External_Tag is the same as
+Y'External_Tag for some other tagged type declared by a different declaration in the partition,
+an implementation may disallow the partition, or raise Program_Error upon the elaboration of either
+type.</p>
<h2 class="head">!ACATS test</h2>
<p class="text">ACATS tests CD30013 and CD30014 test this rule. One could imagine more complex</p>
<p class="text">tests that try various corner cases of this rule (such as allowing multiple</p>
Questions? Ask the ACAA Technical Agent