c++: Fix access checks for __is_assignable and __is_constructible

gcc/

PR c++/94197
* cp/method.c (assignable_expr): Use cp_unevaluated.
(is_xible_helper): Push a non-deferred access check for
the stub objects created by assignable_expr and constructible_expr.

testsuite/

PR c++/94197
* g++.dg/ext/pr94197.C: New.

gcc/cp/ChangeLog

+2020-03-17  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+	gcc/
+
+	PR c++/94197
+	* cp/method.c (assignable_expr): Use cp_unevaluated.
+	(is_xible_helper): Push a non-deferred access check for
+	the stub objects created by assignable_expr and constructible_expr.
+
+	testsuite/
+
+	PR c++/94197
+	* g++.dg/ext/pr94197.C: New.
+
 2020-03-17  Jakub Jelinek  <jakub@redhat.com>
 
 	* pt.c (tsubst): Fix up duplicated word issue in a diagnostic message.

gcc/cp/method.c

@@ -1739,11 +1739,10 @@ check_nontriv (tree *tp, int *, void *)
 static tree
 assignable_expr (tree to, tree from)
 {
-  ++cp_unevaluated_operand;
+  cp_unevaluated cp_uneval_guard;
   to = build_stub_object (to);
   from = build_stub_object (from);
   tree r = cp_build_modify_expr (input_location, to, NOP_EXPR, from, tf_none);
-  --cp_unevaluated_operand;
   return r;
 }
 
@@ -1806,6 +1805,7 @@ constructible_expr (tree to, tree from)
 static tree
 is_xible_helper (enum tree_code code, tree to, tree from, bool trivial)
 {
+  deferring_access_check_sentinel acs (dk_no_deferred);
   if (VOID_TYPE_P (to) || ABSTRACT_CLASS_TYPE_P (to)
       || (from && FUNC_OR_METHOD_TYPE_P (from)
 	  && (TYPE_READONLY (from) || FUNCTION_REF_QUALIFIED (from))))

gcc/testsuite/g++.dg/ext/pr94197.C

+// { dg-do compile { target c++11 } }
+
+template<typename T>
+  T&& declval() noexcept;
+
+template<bool B>
+struct bool_constant
+{
+  static constexpr bool value = B;
+  using type = bool_constant;
+};
+
+using true_type = bool_constant<true>;
+using false_type = bool_constant<false>;
+
+template<bool, typename T, typename Arg>
+  struct __is_nt_constructible_impl
+  : public false_type
+  { };
+
+template<typename T, typename Arg>
+  struct __is_nt_constructible_impl<true, T, Arg>
+  : public bool_constant<noexcept(static_cast<T>(declval<Arg>()))>
+  { };
+
+template<typename T, typename Arg>
+  using __is_nothrow_constructible_impl
+    = __is_nt_constructible_impl<__is_constructible(T, Arg), T, Arg>;
+
+template<typename T>
+  struct __is_nothrow_copy_constructible_impl
+  : public __is_nothrow_constructible_impl<T, const T&>
+  { };
+
+template<typename T>
+  struct is_nothrow_copy_constructible
+  : public __is_nothrow_copy_constructible_impl<T>::type
+  { };
+
+template<bool, typename T, typename Arg>
+  struct __is_nt_assignable_impl
+  : public false_type
+  { };
+
+template<typename T, typename Arg>
+  struct __is_nt_assignable_impl<true, T, Arg>
+  : public bool_constant<noexcept(declval<T&>() = declval<Arg>())>
+  { };
+
+template<typename T, typename Arg>
+  using __is_nothrow_assignable_impl
+    = __is_nt_assignable_impl<__is_assignable(T, Arg), T, Arg>;
+
+template<typename T>
+  struct __is_nothrow_copy_assignable_impl
+  : public __is_nothrow_assignable_impl<T, const T&>
+  { };
+
+template<typename T>
+  struct is_nothrow_copy_assignable
+  : public __is_nothrow_copy_assignable_impl<T>::type
+  { };
+
+struct NType
+{
+  NType();
+private:
+  NType(const NType&);
+  NType& operator=(const NType&);
+};
+
+
+static_assert( !is_nothrow_copy_constructible<NType>::value, "" );
+static_assert( !is_nothrow_copy_assignable<NType>::value, "" );