libstdc++: Make std::compare_three_way check if <=> is valid (PR 93479)

Currently types that cannot be compared using <=> but which are
convertible to pointers will be compared by converting to pointers
first. They should not be comparable.

	PR libstdc++/93479
	* libsupc++/compare (__3way_builtin_ptr_cmp): Require <=> to be valid.
	* testsuite/18_support/comparisons/object/93479.cc: New test.

libstdc++-v3/ChangeLog

 2020-01-29  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/93479
+	* libsupc++/compare (__3way_builtin_ptr_cmp): Require <=> to be valid.
+	* testsuite/18_support/comparisons/object/93479.cc: New test.
+
 	* testsuite/std/ranges/access/end.cc: Do not assume test_range::end()
 	returns the same type as test_range::begin(). Add comments.
 	* testsuite/std/ranges/access/rbegin.cc: Likewise.

libstdc++-v3/libsupc++/compare

@@ -525,7 +525,9 @@ namespace std
     // BUILTIN-PTR-THREE-WAY(T, U)
     template<typename _Tp, typename _Up>
       concept __3way_builtin_ptr_cmp
-	= convertible_to<_Tp, const volatile void*>
+	= requires(_Tp&& __t, _Up&& __u)
+	    { static_cast<_Tp&&>(__t) <=> static_cast<_Up&&>(__u); }
+	  && convertible_to<_Tp, const volatile void*>
 	  && convertible_to<_Up, const volatile void*>
 	  && ! requires(_Tp&& __t, _Up&& __u)
 	     { operator<=>(static_cast<_Tp&&>(__t), static_cast<_Up&&>(__u)); }

libstdc++-v3/testsuite/18_support/comparisons/object/93479.cc

+// Copyright (C) 2020 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/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <compare>
+
+void
+test01()
+{
+  // PR libstdc++/93479
+  int i[1];
+  int j[1];
+  std::compare_three_way{}(i, j); // { dg-error "no match" }
+}
+
+void
+test02()
+{
+  struct X
+  {
+    operator const char*() const noexcept { return "!"; }
+    void operator<=>(X) const = delete;
+    void operator<=>(const void*) const = delete;
+  } x;
+  std::compare_three_way{}(x, x); // { dg-error "no match" }
+  std::compare_three_way{}(x, ""); // { dg-error "no match" }
+  std::compare_three_way{}("", x); // { dg-error "no match" }
+}