c++: Make __is_{,nothrow_}convertible SFINAE on access [PR107049]

The is_convertible built-ins should return false if the conversion fails
an access check, not report an error.

	PR c++/107049

gcc/cp/ChangeLog:

	* method.cc (is_convertible_helper): Use access check sentinel.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_convertible4.C: New test.
	* g++.dg/ext/is_nothrow_convertible4.C: New test.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/is_convertible/requirements/access.cc: New
	test.

gcc/cp/method.cc

@@ -2246,6 +2246,7 @@ is_convertible_helper (tree from, tree to)
     return integer_one_node;
   cp_unevaluated u;
   tree expr = build_stub_object (from);
+  deferring_access_check_sentinel acs (dk_no_deferred);
   return perform_implicit_conversion (to, expr, tf_none);
 }
 

gcc/testsuite/g++.dg/ext/is_convertible4.C

@@ -0,0 +1,33 @@
+// PR c++/107049
+// { dg-do compile { target c++11 } }
+// Failed access check should be a substitution failure, not an error.
+
+template<bool B>
+struct bool_constant { static constexpr bool value = B; };
+
+template<typename From, typename To>
+struct is_convertible
+: public bool_constant<__is_convertible(From, To)>
+{ };
+
+#if __cpp_variable_templates
+template<typename From, typename To>
+constexpr bool is_convertible_v = __is_convertible(From, To);
+#endif
+
+class Private
+{
+  operator int() const
+  {
+    static_assert( not is_convertible<Private, int>::value, "" );
+#if __cpp_variable_templates
+    static_assert( not is_convertible_v<Private, int>, "" );
+#endif
+    return 0;
+  }
+};
+
+static_assert( not is_convertible<Private, int>::value, "" );
+#if __cpp_variable_templates
+static_assert( not is_convertible_v<Private, int>, "" );
+#endif

gcc/testsuite/g++.dg/ext/is_nothrow_convertible4.C

@@ -0,0 +1,33 @@
+// PR c++/107049
+// { dg-do compile { target c++11 } }
+// Failed access check should be a substitution failure, not an error.
+
+template<bool B>
+struct bool_constant { static constexpr bool value = B; };
+
+template<typename From, typename To>
+struct is_nt_convertible
+: public bool_constant<__is_nothrow_convertible(From, To)>
+{ };
+
+#if __cpp_variable_templates
+template<typename From, typename To>
+constexpr bool is_nt_convertible_v = __is_nothrow_convertible(From, To);
+#endif
+
+class Private
+{
+  operator int() const
+  {
+    static_assert( not is_nt_convertible<Private, int>::value, "" );
+#if __cpp_variable_templates
+    static_assert( not is_nt_convertible_v<Private, int>, "" );
+#endif
+    return 0;
+  }
+};
+
+static_assert( not is_nt_convertible<Private, int>::value, "" );
+#if __cpp_variable_templates
+static_assert( not is_nt_convertible_v<Private, int>, "" );
+#endif

libstdc++-v3/testsuite/20_util/is_convertible/requirements/access.cc

@@ -0,0 +1,18 @@
+// { dg-do compile { target  c++11 } }
+// PR c++/107049
+
+#include <type_traits>
+
+class Private
+{
+  operator int() const
+  {
+    static_assert( not std::is_convertible<Private, int>::value, "" );
+#if __cpp_lib_type_trait_variable_templates
+    static_assert( not std::is_convertible_v<Private, int>, "" );
+#endif
+    return 0;
+  }
+};
+
+static_assert( not std::is_convertible<Private, int>::value, "" );