[Bug c/102502] C11: _Static_assert disallows const int operand in -O0 while allows it in higher -O

["i at maskray dot me" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102502

--- Comment #3 from Fangrui Song <i at maskray dot me> ---
OK, Andrew asked me to file it :)
I just wanted to fix glibc and run away from the GCC inconsistency.

I know that
https://www.iso-9899.info/n1570.html#6.6 p10 says
"An implementation may accept other forms of constant expressions."

Accepting `const int` in C mode is an extension, but it seems odd to be
inconsistent (-O0 and -O2 -Wpedantic reject it while -O2 allows it).


% cat reduce.i
const int __alloc_dir_allocation_size = 8;
void __alloc_dir() { _Static_assert(__alloc_dir_allocation_size, ""); }

% gcc reduce.i -c -std=c11 
reduce.i: In function ‘__alloc_dir’:
reduce.i:2:37: error: expression in static assertion is not constant
    2 | void __alloc_dir() { _Static_assert(__alloc_dir_allocation_size, ""); }
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~
% gcc reduce.i -c -std=c11 -O1
% gcc reduce.i -c -std=c11 -O2
% gcc reduce.i -c -std=c11 -O2 -Wpedantic
reduce.i: In function ‘__alloc_dir’:
reduce.i:2:37: warning: expression in static assertion is not an integer
constant expression [-Wpedantic]
    2 | void __alloc_dir() { _Static_assert(__alloc_dir_allocation_size, ""); }
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~


Clang just rejects it in all optimization levels.


% clang reduce.i -c -std=c11 -O0
reduce.i:2:37: error: static_assert expression is not an integral constant
expression
void __alloc_dir() { _Static_assert(__alloc_dir_allocation_size, ""); }
                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
% clang reduce.i -c -std=c11 -O1
reduce.i:2:37: error: static_assert expression is not an integral constant
expression
void __alloc_dir() { _Static_assert(__alloc_dir_allocation_size, ""); }
                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.