libstdc++: Implement C++26 P2927R3 - Inspecting exception_ptr

The following patch attempts to implement the C++26 P2927R3 - Inspecting exception_ptr
paper (but not including P3748R0, I plan to play with it incrementally and
it will really depend on the Constexpr exceptions patch).

The function template is implemented using an out of line private method of
exception_ptr, so that P3748R0 then can use if consteval and provide a
constant evaluation variant of it.

2025-06-26  Jakub Jelinek  <jakub@redhat.com>

	* include/bits/version.def (exception_ptr_cast): Add.
	* include/bits/version.h: Regenerate.
	* libsupc++/exception: Define __glibcxx_want_exception_ptr_cast before
	including bits/version.h.
	* libsupc++/exception_ptr.h (std::exception_ptr_cast): Define.
	(std::__exception_ptr::exception_ptr::_M_exception_ptr_cast): Declare.
	* libsupc++/eh_ptr.cc
	(std::__exception_ptr::exception_ptr::_M_exception_ptr_cast): Define.
	* src/c++23/std.cc.in (std::exception_ptr_cast): Export.
	* config/abi/pre/gnu.ver: Export
	_ZNKSt15__exception_ptr13exception_ptr21_M_exception_ptr_castERKSt9type_info
	at CXXABI_1.3.17.
	* testsuite/util/testsuite_abi.cc (check_version): Allow CXXABI_1.3.17.
	* testsuite/18_support/exception_ptr/exception_ptr_cast.cc: New test.

2 files changed, 82 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/18_support/exception_ptr/exception_ptr_cast.cc b/libstdc++-v3/testsuite/18_support/exception_ptr/exception_ptr_cast.cc
new file mode 100644
index 0000000..6a6fbfe
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/exception_ptr/exception_ptr_cast.cc
@@ -0,0 +1,81 @@
+// { dg-do run { target c++26 } }
+
+// Copyright (C) 2025 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// exception_ptr_cast.
+
+#include <exception>
+#include <testsuite_hooks.h>
+
+#if __cpp_lib_exception_ptr_cast != 202506L
+# error "__cpp_lib_exception_ptr_cast != 202506"
+#endif
+
+struct A { int a; };
+struct B : A {};
+struct C : B {};
+struct D {};
+struct E : virtual C { int e; virtual ~E () {} };
+struct F : virtual E, virtual C { int f; };
+struct G : virtual F, virtual C, virtual E {
+  G () : g (4) { a = 1; e = 2; f = 3; } int g;
+};
+
+void test01()
+{
+  auto a = std::make_exception_ptr(C{ 42 });
+  auto b = std::exception_ptr_cast<C>(a);
+  VERIFY( b != nullptr );
+  VERIFY( b->a == 42 );
+  auto c = std::exception_ptr_cast<B>(a);
+  VERIFY( c == static_cast<const B*>(b) );
+  auto d = std::exception_ptr_cast<A>(a);
+  VERIFY( d == static_cast<const A*>(b) );
+  auto e = std::exception_ptr_cast<D>(a);
+  VERIFY( e == nullptr );
+  auto f = std::make_exception_ptr(42L);
+  auto g = std::exception_ptr_cast<long>(f);
+  VERIFY( g != nullptr );
+  VERIFY( *g == 42L );
+  try
+    {
+      throw G ();
+    }
+  catch (...)
+    {
+      auto h = std::current_exception();
+      auto i = std::exception_ptr_cast<G>(h);
+      VERIFY( i != nullptr );
+      VERIFY( i->a == 1 && i->e == 2 && i->f == 3 && i->g == 4 );
+      auto j = std::exception_ptr_cast<A>(h);
+      VERIFY( j == static_cast<const A*>(i) );
+      auto k = std::exception_ptr_cast<C>(h);
+      VERIFY( k == static_cast<const C*>(i) );
+      auto l = std::exception_ptr_cast<E>(h);
+      VERIFY( l == static_cast<const E*>(i) );
+      auto m = std::exception_ptr_cast<F>(h);
+      VERIFY( m == static_cast<const F*>(i) );
+      auto n = std::exception_ptr_cast<G>(a);
+      VERIFY( n == nullptr );
+    }
+}
+
+int main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/util/testsuite_abi.cc b/libstdc++-v3/testsuite/util/testsuite_abi.cc
index 7bffc6b..4b01023 100644
--- a/libstdc++-v3/testsuite/util/testsuite_abi.cc
+++ b/libstdc++-v3/testsuite/util/testsuite_abi.cc
@@ -241,6 +241,7 @@ check_version(symbol& test, bool added)
 #ifdef __riscv
       known_versions.push_back("CXXABI_1.3.16");
 #endif
+      known_versions.push_back("CXXABI_1.3.17");
       known_versions.push_back("CXXABI_IEEE128_1.3.13");
       known_versions.push_back("CXXABI_TM_1");
       known_versions.push_back("CXXABI_FLOAT128");