libstdc++: Fix BUILTIN-PTR-CMP helpers

The helpers that implement BUILTIN-PTR-CMP do not currently check if the
arguments are actually comparable, so the concept is true when it
shouldn't be.

Since we're trying to test for an unambiguous conversion to pointers, we
can also require that it returns bool, because the built-in comparisons
for pointers do return bool.

	* include/bits/range_cmp.h (__detail::__eq_builtin_ptr_cmp): Require
	equality comparison to be valid and return bool.
	(__detail::__less_builtin_ptr_cmp): Likewise for less-than comparison.
	* testsuite/20_util/function_objects/range.cmp/equal_to.cc: Check
	type with ambiguous conversion to fundamental types.
	* testsuite/20_util/function_objects/range.cmp/less.cc: Likewise.

libstdc++-v3/ChangeLog

+2020-02-09  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/range_cmp.h (__detail::__eq_builtin_ptr_cmp): Require
+	equality comparison to be valid and return bool.
+	(__detail::__less_builtin_ptr_cmp): Likewise for less-than comparison.
+	* testsuite/20_util/function_objects/range.cmp/equal_to.cc: Check
+	type with ambiguous conversion to fundamental types.
+	* testsuite/20_util/function_objects/range.cmp/less.cc: Likewise.
+
 2020-02-07  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/iterator_concepts.h (iter_difference_t, iter_value_t):

libstdc++-v3/include/bits/range_cmp.h

@@ -62,7 +62,8 @@ namespace ranges
     // BUILTIN-PTR-CMP(T, ==, U)
     template<typename _Tp, typename _Up>
       concept __eq_builtin_ptr_cmp
-	= convertible_to<_Tp, const volatile void*>
+	= requires (_Tp&& __t, _Up&& __u) { { __t == __u } -> same_as<bool>; }
+	  && convertible_to<_Tp, const volatile void*>
 	  && convertible_to<_Up, const volatile void*>
 	  && (! requires(_Tp&& __t, _Up&& __u)
 	      { operator==(std::forward<_Tp>(__t), std::forward<_Up>(__u)); }
@@ -73,7 +74,8 @@ namespace ranges
     // BUILTIN-PTR-CMP(T, <, U)
     template<typename _Tp, typename _Up>
       concept __less_builtin_ptr_cmp
-	= convertible_to<_Tp, const volatile void*>
+	= requires (_Tp&& __t, _Up&& __u) { { __t < __u } -> same_as<bool>; }
+	  && convertible_to<_Tp, const volatile void*>
 	  && convertible_to<_Up, const volatile void*>
 	  && (! requires(_Tp&& __t, _Up&& __u)
 	      { operator<(std::forward<_Tp>(__t), std::forward<_Up>(__u)); }

libstdc++-v3/testsuite/20_util/function_objects/range.cmp/equal_to.cc

@@ -69,6 +69,15 @@ test02()
   VERIFY( f(x, x) );
 }
 
+struct Y
+{
+  operator void*() const;
+  operator int() const;
+};
+
+// X{} == X{} is ambiguous so ranges::equal_to{}(X{}, X{}) should be invalid
+static_assert( !std::is_invocable_v<F&, Y, Y> );
+
 int
 main()
 {

libstdc++-v3/testsuite/20_util/function_objects/range.cmp/less.cc

@@ -74,6 +74,15 @@ test02()
   VERIFY( ! f(x, x) );
 }
 
+struct Y
+{
+  operator void*() const;
+  operator int() const;
+};
+
+// X{} == X{} is ambiguous so ranges::less{}(X{}, X{}) should be invalid
+static_assert( !std::is_invocable_v<F&, Y, Y> );
+
 int
 main()
 {