c++: wrong std::is_convertible with cv-qual fn [PR109680]

This PR points out that std::is_convertible has given the wrong answer
in

  static_assert (!std::is_convertible_v <int () const, int (*) ()>, "");

since r13-2822 implemented __is_{,nothrow_}convertible.

std::is_convertible uses the imaginary

  To test() { return std::declval<From>(); }

to do its job.  Here, From is 'int () const'.  std::declval is defined as:

  template<class T>
  typename std::add_rvalue_reference<T>::type declval() noexcept;

std::add_rvalue_reference is defined as "If T is a function type that
has no cv- or ref- qualifier or an object type, provides a member typedef
type which is T&&, otherwise type is T."

In our case, T is cv-qualified, so the result is T, so we end up with

  int () const declval() noexcept;

which is invalid.  In other words, this is pretty much like:

  using T = int () const;
  T fn1(); // bad, fn returning a fn
  T& fn2(); // bad, cannot declare reference to qualified function type
  T* fn3(); // bad, cannot declare pointer to qualified function type

  using U = int ();
  U fn4(); // bad, fn returning a fn
  U& fn5(); // OK
  U* fn6(); // OK

I think is_convertible_helper needs to simulate std::declval better.
To that end, I'm introducing build_trait_object, to be used where
a declval is needed.

	PR c++/109680

gcc/cp/ChangeLog:

	* method.cc (build_trait_object): New.
	(assignable_expr): Use it.
	(ref_xes_from_temporary): Likewise.
	(is_convertible_helper): Likewise.  Check FUNC_OR_METHOD_TYPE_P.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_convertible6.C: New test.

gcc/cp/method.cc

@@ -1902,6 +1902,27 @@ build_stub_object (tree reftype)
   return convert_from_reference (stub);
 }
 
+/* Build a std::declval<TYPE>() expression and return it.  */
+
+tree
+build_trait_object (tree type)
+{
+  /* TYPE can't be a function with cv-/ref-qualifiers: std::declval is
+     defined as
+
+       template<class T>
+       typename std::add_rvalue_reference<T>::type declval() noexcept;
+
+     and std::add_rvalue_reference yields T when T is a function with
+     cv- or ref-qualifiers, making the definition ill-formed.  */
+  if (FUNC_OR_METHOD_TYPE_P (type)
+      && (type_memfn_quals (type) != TYPE_UNQUALIFIED
+	  || type_memfn_rqual (type) != REF_QUAL_NONE))
+    return error_mark_node;
+
+  return build_stub_object (type);
+}
+
 /* Determine which function will be called when looking up NAME in TYPE,
    called with a single ARGTYPE argument, or no argument if ARGTYPE is
    null.  FLAGS and COMPLAIN are as for build_new_method_call.
@@ -2050,8 +2071,8 @@ static tree
 assignable_expr (tree to, tree from)
 {
   cp_unevaluated cp_uneval_guard;
-  to = build_stub_object (to);
-  from = build_stub_object (from);
+  to = build_trait_object (to);
+  from = build_trait_object (from);
   tree r = cp_build_modify_expr (input_location, to, NOP_EXPR, from, tf_none);
   return r;
 }
@@ -2231,7 +2252,9 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
     return false;
   /* We don't check is_constructible<T, U>: if T isn't constructible
      from U, we won't be able to create a conversion.  */
-  tree val = build_stub_object (from);
+  tree val = build_trait_object (from);
+  if (val == error_mark_node)
+    return false;
   if (!TYPE_REF_P (from) && TREE_CODE (from) != FUNCTION_TYPE)
     val = CLASS_TYPE_P (from) ? force_rvalue (val, tf_none) : rvalue (val);
   return ref_conv_binds_to_temporary (to, val, direct_init_p).is_true ();
@@ -2246,7 +2269,15 @@ is_convertible_helper (tree from, tree to)
   if (VOID_TYPE_P (from) && VOID_TYPE_P (to))
     return integer_one_node;
   cp_unevaluated u;
-  tree expr = build_stub_object (from);
+  tree expr = build_trait_object (from);
+  /* std::is_{,nothrow_}convertible test whether the imaginary function
+     definition
+
+       To test() { return std::declval<From>(); }
+
+     is well-formed.  A function can't return a function.  */
+  if (FUNC_OR_METHOD_TYPE_P (to) || expr == error_mark_node)
+    return error_mark_node;
   deferring_access_check_sentinel acs (dk_no_deferred);
   return perform_implicit_conversion (to, expr, tf_none);
 }

gcc/testsuite/g++.dg/ext/is_convertible6.C

@@ -0,0 +1,16 @@
+// PR c++/109680
+// { dg-do compile { target c++11 } }
+
+#define SA(X) static_assert((X),#X)
+
+SA(!__is_convertible(int () const, int (*)()));
+SA(!__is_convertible(int (*)(), int () const));
+
+SA( __is_convertible(int (), int (*)()));
+SA(!__is_convertible(int (*)(), int ()));
+
+SA( __is_convertible(int (int), int (*) (int)));
+SA(!__is_convertible(int (*) (int), int (int)));
+
+SA(!__is_convertible(int (int) const, int (*) (int)));
+SA(!__is_convertible(int (*) (int), int (int) const));