[Bug c++/107104] semantics of __builtin_constant_p within static_assert and return value

[yann at droneaud dot fr via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107104

Yann Droneaud  changed:

   What|Removed |Added

 CC||yann at droneaud dot fr

--- Comment #3 from Yann Droneaud  ---
I'm experiencing the same issue:

#include 

void a(int v)
{
static_assert(__builtin_constant_p(v) && v == -1, "failure");
}

GCC 13 and below complains:

: In function 'a':
:16:43: error: expression in static assertion is not constant
   16 | static_assert(__builtin_constant_p(v) && v == -1, "failure");
  |   ^~
Compiler returned: 1


clang current trunk, eg above 15.0, seems to finally get it right:

:16:5: error: static assertion failed due to requirement
'__builtin_constant_p(v) && v == -1': failure
static_assert(__builtin_constant_p(v) && v == -1, "failure");
^ ~~
/usr/include/assert.h:143:24: note: expanded from macro 'static_assert'
# define static_assert _Static_assert
   ^
1 error generated.
Compiler returned: 1

see https://godbolt.org/z/KKn6xTG8a

[Bug c++/107104] semantics of __builtin_constant_p within static_assert and return value

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107104

--- Comment #2 from Jonathan Wakely  ---
(In reply to Hannes Hauswedell from comment #1)
> It seems that __builtin_constant_p does not indicate whether something *can
> be* a constant but whether *it is* a constant.

Correct.

[Bug c++/107104] semantics of __builtin_constant_p within static_assert and return value

[h2+bugs at fsfe dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107104

Hannes Hauswedell  changed:

   What|Removed |Added

 CC||h2+bugs at fsfe dot org

--- Comment #1 from Hannes Hauswedell  ---
It seems that __builtin_constant_p does not indicate whether something *can be*
a constant but whether *it is* a constant.
If you evaluate it in a non-const-context, the expression passed as argument
may or may not be evaluated at compile-time, so the value of
__builtin_constant_p may or may not be 1.

This example illustrates the behaviour:

```cpp
constexpr void foobar() {}// → test() returns 0 or 1
//consteval void foobar() {}  // → test() returns 1

#define TEST_EXPR (foobar(), 0)

int test() {
static_assert(__builtin_constant_p(TEST_EXPR));
return __builtin_constant_p(TEST_EXPR);
}
```


To enforce evaluation in a const-context, you can define a macro like this:

#define IS_CONSTEXPR(...) std::integral_constant::value