libstdc++: Implement P1328 "Making std::type_info::operator== constexpr"

This feature is present in the C++23 draft.

With Jakub's recent front-end changes we can implement constexpr
equality by comparing the addresses of std::type_info objects. We do not
need string comparisons, because for constant evaluation cases we know
we aren't dealing with std::type_info objects defined in other
translation units.

The ARM EABI requires that the type_info::operator== function can be
defined out-of-line (and suggests that should be the default), but to be
a constexpr function it must be defined inline (at least for C++23
mode). To meet these conflicting requirements we make the inline version
of operator== call a new __equal function when called at runtime. That
is an alias for the non-inline definition of operator== defined in
libsupc++.

libstdc++-v3/ChangeLog:

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.30): Export new symbol for
	ARM EABI.
	* include/bits/c++config (_GLIBCXX23_CONSTEXPR): Define.
	* include/std/version (__cpp_lib_constexpr_typeinfo): Define.
	* libsupc++/tinfo.cc: Add #error to ensure non-inline definition
	is emitted.
	(type_info::__equal): Define alias symbol.
	* libsupc++/typeinfo (type_info::before): Combine different
	implementations into one.
	(type_info::operator==): Likewise. Use address equality for
	constant evaluation. Call __equal for targets that require the
	definition to be non-inline.
	* testsuite/18_support/type_info/constexpr.cc: New test.

1 files changed, 3 insertions, 0 deletions

diff --git a/libstdc++-v3/config/abi/pre/gnu.ver b/libstdc++-v3/config/abi/pre/gnu.ver
index c2f09a9..afd242b 100644
--- a/libstdc++-v3/config/abi/pre/gnu.ver
+++ b/libstdc++-v3/config/abi/pre/gnu.ver
@@ -2424,6 +2424,9 @@ GLIBCXX_3.4.30 {
     # std::__timepunct<char>::_M_am_pm_format(const char**)
     _ZNKSt11__timepunctI[cw]E15_M_am_pm_formatEPPK[cw];
 
+    # Only defined #if ! __GXX_TYPEINFO_EQUALITY_INLINE
+    _ZNKSt9type_info7__equalERKS_;
+
 } GLIBCXX_3.4.29;
 
 # Symbols in the support library (libsupc++) have their own tag.