[Bug c++/102921] error: modification of '' is not a constant expression

[f.heckenbach--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102921

--- Comment #1 from Frank Heckenbach <f.heckenb...@fh-soft.de> ---
The following program, compiled with "-std=c++20" gives this error message; I
don't even understand what it's trying to tell me:

  error: modification of '<temporary>' is not a constant expression

#include <initializer_list>
#include <string_view>

constexpr std::initializer_list <std::string_view> a { "" };

Other compilers seem to have no problem with it.

Digging through the headers, I could trace it (via char_traits and
__constant_string_p) to __builtin_is_constant_evaluated. The following version
gives the same error (but I'm still not sure if the problem is in the compiler
or the headers):

#include <initializer_list>
#include <type_traits>

struct S
{
  constexpr S () noexcept { }
  int a = std::is_constant_evaluated ();
};

constexpr std::initializer_list <S> a { { } };