_Static_assert declaration can not be first of for-triplet

[chumarshal1]

/*
 * (c) Copyright 2016-2021 by Solid Sands B.V.,
 *     Amsterdam, the Netherlands. All rights reserved.
 *     Subject to conditions in the RESTRICTIONS file.
 */
int main()
{
    int i = 4;
    for (_Static_assert (3, "This is a three"); i < 7; i++) {
        i += 2;
    }
 }

clang test.c 
test.c:4:10: error: expected identifier or '('
    for (_Static_assert (3, "This is a three"); i < 7; i++) {
         ^
test.c:4:49: error: expected ';' in 'for' statement specifier
    for (_Static_assert (3, "This is a three"); i < 7; i++) {
                                                ^
2 errors generated.

But gcc can build successfully, is it clang issue?

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[frederick-vs-ja]

See WG14-N2912 6.8.5/3:

(Constraints)
The declaration part of a for statement shall only declare identifiers for objects having storage class auto or register.

AFAIK the requirement hasn't been changed since C99.

Perhaps clang is right and gcc is wrong. But it seems not very clear whether it is allowed to declare no identifier (e.g. the declaration is a static assertion).

[apinski-cavium]

MSVC and ICC also accept this code too (I have not tried pure EDG).

But there is no identifier being defined with _Static_assert as specified by the 6.7.10 (Static assertions/Semantics):
If the value of the constant expression compares unequal to 0, the declaration has no effect.

[frederick-vs-ja]

See WG14-N2912 6.8.5/3:

(Constraints)
The declaration part of a for statement shall only declare identifiers for objects having storage class auto or register.

AFAIK the requirement hasn't been changed since C99.

This sentence is eventually removed in C23 (per N3078 etc.).

#129737 is related. It possibly resolved this issue.

[AaronBallman]

@Sirraide -- this looks like a case we missed with your changes; any interest in tackling this as well?

[Sirraide]

@Sirraide -- this looks like a case we missed with your changes; any interest in tackling this as well?

Ah, sure, I’ll take a look at that; that shouldn’t take too long.