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, 48 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/18_support/type_info/constexpr.cc b/libstdc++-v3/testsuite/18_support/type_info/constexpr.cc
new file mode 100644
index 0000000..07f4fb6
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/type_info/constexpr.cc
@@ -0,0 +1,48 @@
+// { dg-options "-std=gnu++23 -frtti" }
+// { dg-do compile { target c++23 } }
+
+#include <typeinfo>
+
+#ifndef __cpp_lib_constexpr_typeinfo
+# error "Feature-test macro for constexpr typeinfo missing in <typeinfo>"
+#elif __cpp_lib_constexpr_typeinfo != 202106L
+# error "Feature-test macro for constexpr typeinfo has wrong value in <typeinfo>"
+#endif
+
+struct X { };
+
+constexpr bool
+test01()
+{
+  if (typeid(int) == typeid(long))
+    return false;
+
+  if (typeid(int) != typeid(int))
+    return false;
+
+  struct X { virtual ~X() { } };
+
+  if (typeid(X) != typeid(X))
+    return false;
+
+  if (typeid(X) == typeid(::X))
+    return false;
+
+  if (typeid(X) == typeid(int))
+    return false;
+
+  const auto& ti_x = typeid(X);
+  if (ti_x != ti_x)
+    return false;
+
+  if (ti_x != typeid(X))
+    return false;
+
+  struct Y { };
+  if (ti_x == typeid(Y))
+    return false;
+
+  return true;
+}
+
+static_assert( test01() );