GNU C typeof omits side effects of a variably modified operand

[71iq]

Version

Compiler: 7.0, commit 43e10b90fc83ed2a436fb79b241be8dbf3dc6846
Platform: Linux x86_64 (LP64)
Reference/compiler for generated C: GCC 13.3.0
Preset: linux-gcc-debug, with EDG_CPFE_USE_ASAN=OFF

Relevant configuration:

#define BACK_END_IS_C_GEN_BE 1
#define C_GEN_BE_GENERATES_ANSI_C 1
#define GNU_EXTENSIONS_ALLOWED 1
#define TARG_CHAR_BIT 8
#define TARG_SIZEOF_INT 4
#define TARG_SIZEOF_LONG 8
#define TARG_SIZEOF_POINTER 8

Code to reproduce

#include <assert.h>

int main(void)
{
    int k = 1;
    int a[2];
    typeof (++k, (int (*)[k])a) p;
    (void)sizeof(p);
    assert(k == 2);
}

Compiler options:

--c --c11 --no_strict_gnu --gnu_version=130300 --gen_c_file_name typeof.c.int.c typeof.c.i

Behavior

The typeof operand has variably modified type: a pointer to a variable-length array. GNU C evaluates such an operand for its side effects, so ++k should execute and the assertion should pass.

The reference program compiled with GCC exits with status 0. The translated program aborts:

Assertion `k == 2' failed.

The generated C contains no increment of k; its value remains 1.

This concerns variably modified operands. An ordinary typeof(++k) whose type is int should remain unevaluated. The positive initial bound and use of sizeof(p) avoid a zero-sized VLA or reading an uninitialized pointer.

Reference: GCC typeof documentation.

The attachment contains the full --dump_configuration output and self-contained reproduction inputs.

Source, generated C, results, and full configuration

Reporting acknowledgements

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• I have checked open issues and existing discussions before filing this report.
• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
• I have reduced the reproducing source code to the best of my ability.
• I have selected at least one standard configuration or uploaded a non-standard configuration dump.
• I understand code shared via this form is NOT confidential.
• I have permission to share the reproducing source code and grant permission for the project to use the source code (or derivatives of the source code) to create tests.
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