libstdc++: Add static assertions to futures and promises [LWG 3458]

LWG recently decided it should be ill-formed to instantiate std::future
and std::shared_future for types that can't be returned from a function.
This adds static assertions to enforce it (std::future already failed,
but this makes the error more understandable).

LWG 3466 extends that to std::promise. The actual constraint is that
t.~T() is well-formed for the primary template, but rejecting arrays and
functions as done for futures matches that condition.

libstdc++-v3/ChangeLog:

	* include/std/future (future, shared_future, promise): Add
	static assertions to the primary template to reject array and
	function types.
	* testsuite/30_threads/future/requirements/lwg3458.cc: New test.
	* testsuite/30_threads/promise/requirements/lwg3466.cc: New test.
	* testsuite/30_threads/shared_future/requirements/lwg3458.cc: New test.

libstdc++-v3/include/std/future

@@ -763,6 +763,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Res>
     class future : public __basic_future<_Res>
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3458. Is shared_future intended to work with arrays or function types?
+      static_assert(!is_array<_Res>{}, "result type is not an array");
+      static_assert(!is_function<_Res>{}, "result type is not a function");
+
       friend class promise<_Res>;
       template<typename> friend class packaged_task;
       template<typename _Fn, typename... _Args>
@@ -893,6 +898,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Res>
     class shared_future : public __basic_future<_Res>
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3458. Is shared_future intended to work with arrays or function types?
+      static_assert(!is_array<_Res>{}, "result type is not an array");
+      static_assert(!is_function<_Res>{}, "result type is not a function");
+
       typedef __basic_future<_Res> _Base_type;
 
     public:
@@ -1045,6 +1055,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Res>
     class promise
     {
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3466: Specify the requirements for promise/future/[...] consistently
+      static_assert(!is_array<_Res>{}, "result type is not an array");
+      static_assert(!is_function<_Res>{}, "result type is not a function");
+
       typedef __future_base::_State_base 	_State;
       typedef __future_base::_Result<_Res>	_Res_type;
       typedef __future_base::_Ptr<_Res_type>	_Ptr_type;

libstdc++-v3/testsuite/30_threads/future/requirements/lwg3458.cc

@@ -0,0 +1,34 @@
+// 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/>.
+
+// { dg-do compile { target c++11 } }
+
+// LWG 3458
+// Is shared_future intended to work with arrays or function types?
+
+#include <future>
+
+std::future<int(&)[1]> good;
+std::future<int(&)()> good2;
+
+std::future<int[1]> bad; // { dg-error "here" }
+// { dg-error "result type is not an array" "" { target *-*-* } 0 }
+// { dg-prune-output "function returning an array" }
+
+std::future<int()> bad2; // { dg-error "here" }
+// { dg-error "result type is not a function" "" { target *-*-* } 0 }
+// { dg-prune-output "function returning a function" }

libstdc++-v3/testsuite/30_threads/promise/requirements/lwg3466.cc

@@ -0,0 +1,34 @@
+// 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/>.
+
+// { dg-do compile { target c++11 } }
+
+// LWG 3466
+// Specify the requirements for promise/future/shared_future consistently
+
+#include <future>
+
+std::promise<int(&)[1]> good;
+std::promise<int(&)()> good2;
+
+std::promise<int[1]> bad; // { dg-error "here" }
+// { dg-error "result type is not an array" "" { target *-*-* } 0 }
+// { dg-prune-output {request for member '~int \[1\]'} }
+
+std::promise<int()> bad2; // { dg-error "here" }
+// { dg-error "result type is not a function" "" { target *-*-* } 0 }
+// { dg-prune-output {'sizeof \(int\(\)\)'} }

libstdc++-v3/testsuite/30_threads/shared_future/requirements/lwg3458.cc

@@ -0,0 +1,32 @@
+// 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/>.
+
+// { dg-do compile { target c++11 } }
+
+// LWG 3458
+// Is shared_future intended to work with arrays or function types?
+
+#include <future>
+
+std::shared_future<int(&)[1]> good;
+std::shared_future<int(&)()> good2;
+
+std::shared_future<int[1]> bad; // { dg-error "here" }
+// { dg-error "result type is not an array" "" { target *-*-* } 0 }
+
+std::shared_future<int()> bad2; // { dg-error "here" }
+// { dg-error "result type is not a function" "" { target *-*-* } 0 }