https://eel.is/c++draft/basic.types.general#9.sentence-1 says that
std::meta::info and its cv-qualified versions are scalar types too
(and in https://eel.is/c++draft/basic.fundamental#19.sentence-1
that they are fundamental types too).
Now, on the reflection side, eval_is_scalar_type is handled
in the compiler and uses SCALAR_TYPE_P (type) which includes
REFLECTION_TYPE_P check and eval_is_fundamental_type includes that
explicitly too.
std::is_fundamental uses
template<typename _Tp>
struct is_fundamental
: public __or_<is_arithmetic<_Tp>, is_void<_Tp>,
is_null_pointer<_Tp>
#if __cpp_impl_reflection >= 202506L
, is_reflection<_Tp>
#endif
>::type
{ };
but for std::is_scalar we apparently forgot to include is_reflection.
The following patch fixes that.
2026-04-26 Jakub Jelinek <jakub@redhat.com>
PR libstdc++/125024
* include/std/type_traits (std::is_scalar): For
__cpp_impl_reflection >= 202506L handle is_reflection types as
scalar.
* testsuite/20_util/is_scalar/reflection.cc: New test.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
7f4cc8140e