Submitter: Joseph Myers (UK)
Submission Date: 2007-03-24
Source: Joseph Myers <joseph@codesourcery.com>
Reference Document: ISO/IEC WG14
N1223
Version: 1.0
Date: 2007-03-24
Subject: VLAs and conditional expressions
Summary
Consider the code:
int a, b;
void *p1(void), *p2(void);
int c1(void);
int d1(void);
int z1(void), z2(void);
int
h(void)
{
int r = (c1()
? (z1(), (int (*)[d1()])p1())
: (z2(), (int (*)[])p2()))[a][b];
return r;
}
The type of the conditional expression involves the size expression
d1() that's only evaluated in one part of the expression,
and this information is needed to evaluate the array reference even
when c1() returns false.
For a more complicated example and discussion see reflector messages 10731-10754. The validity of that more complicated example depends on the interpretation of composite type rules as in DR#XXX-2, so this example has been simplified to avoid that problem.
Suggested Technical Corrigendum
6.7.5.2 pargraph 6 at end add "or one of the size specifiers (including
the case of a single size specifier where the other array type does not
include a size specifier) is not an integer constant expression and is not
evaluated during the flow of execution." with a footnote "This case
arises where a conditional expression involves a cast to variably
modified type or a compound literal of variably modified type."