!standard 4.4 (3)                                 09-06-07    AI05-0158-1/01
!standard 4.5.2(3)
!standard 4.5.2(30/2)
!class Amendment 09-06-07
!status work item 09-06-07
!status received 09-03-30
!priority Low
!difficulty Medium
!subject Generalizing membership tests

!summary

Extend the syntax of membership tests to simplify complex conditions that can
be expressed as membership in a subset of values of any type.

!problem

Conditions of the form  (X = A) or else (X = B) or else (X = C) where A, B, C 
are of some arbitrary type are common, and will be more frequent when
pre- and post-conditions become widely used. If the values of A, B,and C are
contiguous values of some discrete type, the expression can be
written as a membership operation: (X in A..C) .  Otherwise this syntactic
shortcut is not available. We propose a simple extension of the syntax of
membership operations, so that the right operand can specify a subset of values
of some type.

!proposal

Extend the syntax of memberships as shown in the wording.

!wording

Modify 4.4(3) as follows:

relation ::=
   simple_expression [not] in choice_list
   choice_list ::= choice {'|' choice}
   choice ::= simple_expression | range | subtype_mark

[Editor's comment: This grammer is ambiguous if the expression is used
in a discrete_choice_list (that is, in a variant part, case statement,
or array aggregate. How that is going to be addressed needs to be described.]

Add after 4.5.2(3): (Name Resolution Rules)

If the choice_list in a membership operation has more that one choice, then
all choices must have a single common type, which is the tested type of the
operation.

Add somewhere in 4.5.2(28-31): (Dynamic Semantics)

The left operand is elaborated first.
If the choice_list has more that one choice, the operation is equivalent to a
short-circuit sequence of tests : if a choice is a simple expression the
corresponding test is an equality operation, otherwise it is a membership test
whose right operand is the corresponding range or subtype_mark.

[Editor's comment: The first sentence contradicts 4.5.2(27). Is that really
what we want? We ought to reword this to fit in with the existing wording.]

!discussion

!example

!ACATS test

Add an ACATS test for this new feature.

!appendix

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