[PATCH] c++: Fix access checking for __is_assignable and __is_constructible (c++/94197)

[Ville Voutilainen via Gcc-patches]

Tested on Linux-PPC64.

This ain't no regression. But it seems to hamper attempts to fix library
regressions (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94033).

2020-03-16  Ville Voutilainen  <ville.voutilai...@gmail.com>

    gcc/

    PR c++/94197
    * cp/method.c (assignable_expr, constructible_expr): Push
    a deferred access check for the stub object.

    testsuite/

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

diff --git a/gcc/cp/method.c b/gcc/cp/method.c
index 790d5704092..b429ea48049 100644
--- a/gcc/cp/method.c
+++ b/gcc/cp/method.c
@@ -1739,11 +1739,11 @@ check_nontriv (tree *tp, int *, void *)
 static tree
 assignable_expr (tree to, tree from)
 {
-  ++cp_unevaluated_operand;
+  cp_unevaluated cp_uneval_guard;
+  deferring_access_check_sentinel acs (dk_no_deferred);
   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;
 }
 
@@ -1759,6 +1759,7 @@ constructible_expr (tree to, tree from)
 {
   tree expr;
   cp_unevaluated cp_uneval_guard;
+  deferring_access_check_sentinel acs (dk_no_deferred);
   if (CLASS_TYPE_P (to))
     {
       tree ctype = to;
diff --git a/gcc/testsuite/g++.dg/ext/pr94197.C b/gcc/testsuite/g++.dg/ext/pr94197.C
new file mode 100644
index 00000000000..768bfbac0a6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/pr94197.C
@@ -0,0 +1,75 @@
+// { dg-do compile }
+// { dg-options "-std=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, "" );