PR libstdc++/80762 avoid ambiguous __constructible_from<void, void>

Ensure we don't try to instantiate __is_constructible_from<void, void>,
because there are two partial specializations that are equally good
matches.

	PR libstdc++/80762
	* include/bits/fs_path.h (path::_Path): Use remove_cv_t and is_void.
	* include/experimental/bits/fs_path.h (path::_Path): Likewise.
	* testsuite/27_io/filesystem/path/construct/80762.cc: New test.
	* testsuite/experimental/filesystem/path/construct/80762.cc: New test.

From-SVN: r267056

libstdc++-v3/ChangeLog

+2018-12-12  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/80762
+	* include/bits/fs_path.h (path::_Path): Use remove_cv_t and is_void.
+	* include/experimental/bits/fs_path.h (path::_Path): Likewise.
+	* testsuite/27_io/filesystem/path/construct/80762.cc: New test.
+	* testsuite/experimental/filesystem/path/construct/80762.cc: New test.
+
 2018-12-12  Jakub Jelinek  <jakub@redhat.com>
 
 	P0595R2 - is_constant_evaluated

libstdc++-v3/include/bits/fs_path.h

@@ -110,7 +110,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
     template<typename _Tp1, typename _Tp2 = void>
       using _Path = typename
-	std::enable_if<__and_<__not_<is_same<_Tp1, path>>,
+	std::enable_if<__and_<__not_<is_same<remove_cv_t<_Tp1>, path>>,
+			      __not_<is_void<_Tp1>>,
 			      __constructible_from<_Tp1, _Tp2>>::value,
 		       path>::type;
 

libstdc++-v3/include/experimental/bits/fs_path.h

@@ -125,7 +125,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 
     template<typename _Tp1, typename _Tp2 = void>
       using _Path = typename
-	std::enable_if<__and_<__not_<is_same<_Tp1, path>>,
+	std::enable_if<__and_<__not_<is_same<typename remove_cv<_Tp1>::type,
+					     path>>,
+			      __not_<is_void<_Tp1>>,
 			      __constructible_from<_Tp1, _Tp2>>::value,
 		       path>::type;
 

libstdc++-v3/testsuite/27_io/filesystem/path/construct/80762.cc

+// Copyright (C) 2018 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-options "-std=gnu++17" }
+// { dg-do compile { target c++17 } }
+
+#include <filesystem>
+
+using std::filesystem::path;
+
+static_assert( !std::is_constructible_v<path, void> );
+static_assert( !std::is_constructible_v<path, volatile path> );
+static_assert( !std::is_constructible_v<path, volatile path&> );
+static_assert( !std::is_constructible_v<path, const volatile path> );
+static_assert( !std::is_constructible_v<path, const volatile path&> );

libstdc++-v3/testsuite/experimental/filesystem/path/construct/80762.cc

+// Copyright (C) 2018 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 } }
+
+#include <experimental/filesystem>
+
+using std::experimental::filesystem::path;
+
+static_assert( !std::is_constructible<path, void>::value, "" );
+static_assert( !std::is_constructible<path, volatile path>::value, "" );
+static_assert( !std::is_constructible<path, volatile path&>::value, "" );
+static_assert( !std::is_constructible<path, const volatile path>::value, "" );
+static_assert( !std::is_constructible<path, const volatile path&>::value, "" );