c++: make __is_constructible work with paren-init of aggrs [PR94149]

In C++20 this is well-formed:

  using T = int[2];
  T t(1, 2);

which means that std::is_constructible_v<int[2], int, int> should be true.
But constructible_expr immediately returned the error_mark_node when it
saw a list with more than one element.  To give accurate results in
C++20, we have to try initializing the aggregate from a parenthesized list of
values.

To not repeat the same mistake as in c++/93790, if there's only one
element, I'm trying {} only when () didn't succeed.  is_constructible5.C
verifies this.

In paren-init24.C std::is_nothrow_constructible_v doesn't work due to
 error: invalid 'static_cast' from type 'int' to type 'int [1]'
and
 error: functional cast to array type 'int [2]'

This needs to be fixed in libstdc++.

	PR c++/94149
	* method.c (constructible_expr): In C++20, try using parenthesized
	initialization of aggregates to determine the result of
	__is_constructible.

	* g++.dg/cpp2a/paren-init24.C: New test.
	* g++.dg/cpp2a/paren-init25.C: New test.
	* g++.dg/ext/is_constructible5.C: New test.

gcc/cp/ChangeLog

+2020-04-10  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94149
+	* method.c (constructible_expr): In C++20, try using parenthesized
+	initialization of aggregates to determine the result of
+	__is_constructible.
+
 2020-04-10  Bin Cheng  <bin.cheng@linux.alibaba.com>
 
 	* coroutines.cc (co_await_expander): Simplify.

gcc/cp/method.c

@@ -1799,12 +1799,48 @@ constructible_expr (tree to, tree from)
     {
       if (from == NULL_TREE)
 	return build_value_init (strip_array_types (to), tf_none);
-      else if (TREE_CHAIN (from))
-	return error_mark_node; // too many initializers
+      const int len = list_length (from);
+      if (len > 1)
+	{
+	  if (cxx_dialect < cxx2a)
+	    /* Too many initializers.  */
+	    return error_mark_node;
+
+	  /* In C++20 this is well-formed:
+	       using T = int[2];
+	       T t(1, 2);
+	     which means that std::is_constructible_v<int[2], int, int>
+	     should be true.  */
+	  vec<constructor_elt, va_gc> *v;
+	  vec_alloc (v, len);
+	  for (tree t = from; t; t = TREE_CHAIN (t))
+	    {
+	      tree stub = build_stub_object (TREE_VALUE (t));
+	      constructor_elt elt = { NULL_TREE, stub };
+	      v->quick_push (elt);
+	    }
+	  from = build_constructor (init_list_type_node, v);
+	  CONSTRUCTOR_IS_DIRECT_INIT (from) = true;
+	  CONSTRUCTOR_IS_PAREN_INIT (from) = true;
+	}
+      else
 	from = build_stub_object (TREE_VALUE (from));
       expr = perform_direct_initialization_if_possible (to, from,
 							/*cast*/false,
 							tf_none);
+      /* If t(e) didn't work, maybe t{e} will.  */
+      if (expr == NULL_TREE
+	  && len == 1
+	  && cxx_dialect >= cxx2a)
+	{
+	  from = build_constructor_single (init_list_type_node, NULL_TREE,
+					   from);
+	  CONSTRUCTOR_IS_DIRECT_INIT (from) = true;
+	  CONSTRUCTOR_IS_PAREN_INIT (from) = true;
+	  expr = perform_direct_initialization_if_possible (to, from,
+							    /*cast*/false,
+							    tf_none);
+	}
     }
   return expr;
 }

gcc/testsuite/ChangeLog

+2020-04-10  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94149
+	* g++.dg/cpp2a/paren-init24.C: New test.
+	* g++.dg/cpp2a/paren-init25.C: New test.
+	* g++.dg/ext/is_constructible5.C: New test.
+
 2020-04-10  Fritz Reese  <foreese@gcc.gnu.org>
 
 	* gfortran.dg/asynchronous_5.f03: Fix typo in testcase and add

gcc/testsuite/g++.dg/cpp2a/paren-init24.C

+// PR c++/94149 - make __is_constructible work with paren-init of aggrs.
+// { dg-do compile { target c++2a } }
+
+#include <type_traits>
+
+int main()
+{
+  using T = int[1];
+  T t(1);
+
+  static_assert(__is_constructible(T, int));
+  static_assert(!__is_constructible(T, int, int));
+  static_assert(std::is_constructible_v<T, int>);
+  //FIXME: libstdc++ problem?
+  //static_assert(std::is_nothrow_constructible_v<T, int>);
+
+  using T2 = int[2];
+  T2 t2(1);
+  T2 t3(1, 2);
+
+  static_assert(__is_constructible(T2, int));
+  static_assert(__is_constructible(T2, int, int));
+  static_assert(std::is_constructible_v<T2, int, int>);
+  // FIXME libstdc++ problem?
+  //static_assert(std::is_nothrow_constructible_v<T2, int, int>);
+}

gcc/testsuite/g++.dg/cpp2a/paren-init25.C

+// PR c++/94149 - make __is_constructible work with paren-init of aggrs.
+// { dg-do compile { target c++2a } }
+
+struct nonaggr {
+  nonaggr() {}
+  int i;
+  int j;
+};
+
+struct aggr {
+  int i;
+  int j;
+};
+
+static_assert(__is_constructible(aggr, int, int));
+static_assert(__is_constructible(aggr, int));
+static_assert(!__is_constructible(nonaggr, int, int));
+
+using T = aggr[2];
+static_assert(__is_constructible(T, aggr));
+static_assert(__is_constructible(T, aggr, aggr));
+
+using N = nonaggr[2];
+static_assert(__is_constructible(N, nonaggr));
+static_assert(__is_constructible(N, nonaggr, nonaggr));

gcc/testsuite/g++.dg/ext/is_constructible5.C

+// PR c++/94149 - make __is_constructible work with paren-init of aggrs.
+// { dg-do compile { target c++11 } }
+
+struct S { };
+
+struct W {
+  S& r;
+  W(S& r_) : r(r_) {}
+  operator S&() { return r; }
+};
+
+S s;
+W w(s);
+S& s2(w);
+
+static_assert(__is_constructible(S&, W), "");