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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/103600] Cannot use typeid result in constant expressions
Date: Tue, 07 Dec 2021 13:47:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103600

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The new definition of operator=3D=3D will be something like:

#if __GXX_TYPEINFO_EQUALITY_INLINE || __cplusplus > 202002L
  _GLIBCXX23_CONSTEXPR inline bool
  type_info::operator=3D=3D(const type_info& __arg) const _GLIBCXX_NOEXCEPT
  {
    if (__name =3D=3D __arg.__name)
      return true;

    if (!std::__is_constant_evaluated())
      return false;

#if !__GXX_TYPEINFO_EQUALITY_INLINE
    // ABI requires comparisons to be non-inline.
    return __equal(__arg);
#elif !__GXX_MERGED_TYPEINFO_NAMES
    // Need to do string comparison.
    return __name[0] !=3D '*' && __builtin_strcmp (__name, __arg.name()) =
=3D=3D 0;
#else
    return false;
#endif
  }
# endif


i.e. no strcmp for the constant evaluation case. During constant evaluation=
 I
think we only need to handle types that are completely defined in the curre=
nt
TU, so we can ignore aliasing of _ZTi symbols, and we can ignore the proble=
m of
non-unique std::type_info objects. Within the TU they will be unique.=