libstdc++: Support arrays in std::is_nothrow_constructible (PR 94149)

The front end now supports parenthesized initialization for arrays in
C++20, so extend std::is_nothrow_constructible to support them too.

gcc/testsuite:

	PR c++/94149
	* g++.dg/cpp2a/paren-init24.C: Fix FIXMEs.

libstdc++-v3:

	PR c++/94149
	* include/std/type_traits (__is_nt_constructible_impl): Add partial
	specializations for bounded arrays with non-empty initializers.
	* testsuite/20_util/is_nothrow_constructible/value_c++20.cc: New test.

3 files changed, 88 insertions, 0 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index d09d114d948..80c14a7f45b 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2020-04-21  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR c++/94149
+	* include/std/type_traits (__is_nt_constructible_impl): Add partial
+	specializations for bounded arrays with non-empty initializers.
+	* testsuite/20_util/is_nothrow_constructible/value_c++20.cc: New test.
+
 2020-04-20  Thomas Rodgers  <trodgers@redhat.com>
 
 	* testsuite/lib/libstdc++.exp: Add additional_flags=
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 65b9902b56d..f96b5297b83 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -986,6 +986,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public __bool_constant<noexcept(typename remove_all_extents<_Tp>::type())>
     { };
 
+#if __cpp_aggregate_paren_init
+  template<typename _Tp, size_t _Num, typename _Arg>
+    struct __is_nt_constructible_impl<true, _Tp[_Num], _Arg>
+    : public __is_nt_constructible_impl<true, _Tp, _Arg>
+    { };
+
+  template<typename _Tp, size_t _Num, typename... _Args>
+    struct __is_nt_constructible_impl<true, _Tp[_Num], _Args...>
+    : public __and_<__is_nt_constructible_impl<true, _Tp, _Args>...>
+    { };
+#endif
+
   template<typename _Tp, typename... _Args>
     using __is_nothrow_constructible_impl
       = __is_nt_constructible_impl<__is_constructible(_Tp, _Args...),
diff --git a/libstdc++-v3/testsuite/20_util/is_nothrow_constructible/value_c++20.cc b/libstdc++-v3/testsuite/20_util/is_nothrow_constructible/value_c++20.cc
new file mode 100644
index 00000000000..6bf0a513b14
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/is_nothrow_constructible/value_c++20.cc
@@ -0,0 +1,69 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <type_traits>
+
+static_assert( std::is_nothrow_constructible_v<int[1]> );
+static_assert( std::is_nothrow_constructible_v<int[1], int> );
+static_assert( std::is_nothrow_constructible_v<int[2], int> );
+static_assert( std::is_nothrow_constructible_v<int[2], int, int> );
+static_assert( ! std::is_nothrow_constructible_v<int[1], int, int> );
+static_assert( ! std::is_nothrow_constructible_v<int[]> );
+static_assert( ! std::is_nothrow_constructible_v<int[], int> );
+static_assert( ! std::is_nothrow_constructible_v<int[], int, int> );
+
+struct X
+{
+  X() = default;
+  X(int) noexcept { }
+  X(double) { }
+};
+
+static_assert( std::is_nothrow_constructible_v<X[2]> );
+static_assert( std::is_nothrow_constructible_v<X[1], X> );
+static_assert( std::is_nothrow_constructible_v<X[1], int> );
+static_assert( ! std::is_nothrow_constructible_v<X[1], double> );
+static_assert( ! std::is_nothrow_constructible_v<X[2], int, double> );
+
+struct Y
+{
+  int i;
+  X x;
+};
+
+static_assert( std::is_nothrow_constructible_v<Y> );
+static_assert( std::is_nothrow_constructible_v<Y, Y> );
+static_assert( std::is_nothrow_constructible_v<Y, int> );
+static_assert( ! std::is_nothrow_constructible_v<Y, X> );
+static_assert( std::is_nothrow_constructible_v<Y, int, X> );
+static_assert( std::is_nothrow_constructible_v<Y, int, int> );
+static_assert( ! std::is_nothrow_constructible_v<Y, int, double> );
+
+struct Z : Y { };
+
+static_assert( std::is_nothrow_constructible_v<Z> );
+static_assert( std::is_nothrow_constructible_v<Z, Z> );
+static_assert( std::is_nothrow_constructible_v<Z, Y> );
+static_assert( ! std::is_nothrow_constructible_v<Z, int> );
+static_assert( ! std::is_nothrow_constructible_v<Z, int, X> );
+static_assert( ! std::is_nothrow_constructible_v<Z, int, int> );
+static_assert( ! std::is_nothrow_constructible_v<Z, Y, double> );
+static_assert( ! std::is_nothrow_constructible_v<Z, int, double> );
+static_assert( ! std::is_nothrow_constructible_v<Z, X> );