[Bug c++/87999] New: Constexpr eval. in static_assert makes string_view comparison non constexpr

[wieichdashasse at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87999

            Bug ID: 87999
           Summary: Constexpr eval. in static_assert makes string_view
                    comparison non constexpr
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wieichdashasse at gmail dot com
  Target Milestone: ---

Hey,

I see an inconsistency when using gcc compared to other compilers when using a
constexpr return directly in a (constexpr) static_assert

================================
Minimal repro:

#include <string_view>

constexpr bool IsValid_impl
                (std::string_view name)
{
    return name == "Steve";
}

template<typename F>
constexpr void IsValid(F func)
{
    // Using it via intermediate will make it work
    // constexpr auto tmp = IsValid_impl(func());
    // static_assert(tmp);
    static_assert(IsValid_impl(func()));
}

int main()
{
    IsValid([]{ return "Steve";});
}

(as featured in 
https://godbolt.org/z/YCVMRp)

================================
Output:
test.cpp: In instantiation of 'constexpr void IsValid(F) [with F =
main()::<lambda()>]':

prog.cpp: In instantiation of 'constexpr void IsValid(F) [with F =
main()::<lambda()>]':
prog.cpp:20:46:   required from here
prog.cpp:15:31: error: non-constant condition for static assertion
   15 |     static_assert(IsValid_impl(func()));
      |                   ~~~~~~~~~~~~^~~~~~~~


Adding the intermediate makes it work, hence i think it is a bug.
I've tested this on 8.2.0, 9.0, 7.3.0 and 7.2.0

Cheers,
Justin Meyer