[Bug c/114831] New: typeof doesn't evaluate expression when it has variably modified type in some cases

[Bug c/114831] New: typeof doesn't evaluate expression when it has variably modified type in some cases

[mforney at mforney dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114831

            Bug ID: 114831
           Summary: typeof doesn't evaluate expression when it has
                    variably modified type in some cases
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mforney at mforney dot org
  Target Milestone: ---

C23 introduced typeof, and 6.7.2.5p4 says

> If the type of the operand is a variably modified type, the operand
> is evaluated; otherwise, the operand is not evaluated.

However, gcc doesn't evaluate the operand when it has variably modified type in
some cases.

Consider the following program:

#include <stdio.h>
int n = 1;
int main(void) {
        int a[n], (*p)[n] = 0;
        typeof(puts("&a is variably-modified"), &a) p1;
        typeof(puts("p is variably-modified"), p) p2;
        typeof(puts("p1 is variably-modified"), p1) p3;
        typeof(puts("p2 is variably-modified"), p2) p4;
}

All four of the typeof operands have the same type, int (*)[n]. But when I run
this program it prints just:

p is variably-modified
p2 is variably-modified

[Bug c/114831] typeof doesn't evaluate expression when it has variably modified type in some cases

[uecker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114831

uecker at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-05-18
                 CC|                            |uecker at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from uecker at gcc dot gnu.org ---
Confirmed. 

We need to propagate C_TYPE_VARIABLY_MODIFIED correctly for address of an
array-to-pointer-decay.

[Bug c/114831] typeof doesn't evaluate expression when it has variably modified type in some cases

[muecker at gwdg dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114831

Martin Uecker <muecker at gwdg dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |muecker at gwdg dot de

--- Comment #2 from Martin Uecker <muecker at gwdg dot de> ---

PATCH: https://gcc.gnu.org/pipermail/gcc-patches/2024-May/652089.html

[Bug c/114831] typeof doesn't evaluate expression when it has variably modified type in some cases

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114831

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Martin Uecker <uecker@gcc.gnu.org>:

https://gcc.gnu.org/g:9f1798c1a93257526196a3c19828e40fb28ac551

commit r15-825-g9f1798c1a93257526196a3c19828e40fb28ac551
Author: Martin Uecker <uecker@tugraz.at>
Date:   Sat May 18 14:40:02 2024 +0200

    c: Fix for some variably modified types not being recognized [PR114831]

    We did not evaluate expressions with variably modified types correctly
    in typeof and did not produce warnings when jumping over declarations
    using typeof.  After addressof or array-to-pointer decay we construct
    new pointer types that have to be marked variably modified if the pointer
    target is variably modified.

    2024-05-18 Martin Uecker  <uecker@tugraz.at>

            PR c/114831
    gcc/c/
            * c-typeck.cc (array_to_pointer_conversion, build_unary_op):
            Propagate flag to pointer target.

    gcc/testsuite/
            * gcc.dg/pr114831-1.c: New test.
            * gcc.dg/pr114831-2.c: New test.
            * gcc.dg/gnu23-varmod-1.c: New test.
            * gcc.dg/gnu23-varmod-2.c: New test.