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

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

[me at inclyc dot cn]

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

            Bug ID: 107104
           Summary: semantics of __builtin_constant_p within static_assert
                    and return value
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: me at inclyc dot cn
  Target Milestone: ---

https://godbolt.org/z/oz4e7bobT
https://godbolt.org/z/hE6x9G49q

illustrate different occasions about this issue. 

Consider passing the same operand __builtin_constant_p in static assertions and
return value, expression with in static assertion passed, but the same
expression evaluates to 0 as the return value.

Are there some different scopes / contexts that causes different result?

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91158
Link: https://github.com/llvm/llvm-project/issues/58078

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

[h2+bugs at fsfe dot org]

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

Hannes Hauswedell <h2+bugs at fsfe dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |h2+bugs at fsfe dot org

--- Comment #1 from Hannes Hauswedell <h2+bugs at fsfe dot org> ---
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<bool,
__builtin_constant_p((__VA_ARGS__, 0))>::value

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

[redi at gcc dot gnu.org]

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

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(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

[yann at droneaud dot fr]

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

Yann Droneaud <yann at droneaud dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yann at droneaud dot fr

--- Comment #3 from Yann Droneaud <yann at droneaud dot fr> ---
I'm experiencing the same issue:

    #include <assert.h>

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

GCC 13 and below complains:

    <source>: In function 'a':
    <source>: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:

    <source>: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