libstdc++: Add workaround for weird std::tuple error [PR 96592]

This "fix" makes no sense, but it avoids an error from G++ about
std::is_constructible being incomplete. The real problem is elsewhere,
but this "fixes" the regression for now.

libstdc++-v3/ChangeLog:

	PR libstdc++/96592
	* include/std/tuple (_TupleConstraints<true, T...>): Use
	alternative is_constructible instead of std::is_constructible.
	* testsuite/20_util/tuple/cons/96592.cc: New test.

2 files changed, 62 insertions, 0 deletions

diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
index 1c22d4d..06f5633 100644
--- a/libstdc++-v3/include/std/tuple
+++ b/libstdc++-v3/include/std/tuple
@@ -539,6 +539,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<bool, typename... _Types>
     struct _TupleConstraints
     {
+      template<typename _Tp, typename _Up> // Workaround for PR 96592
+	using is_constructible
+	  = __bool_constant<__is_constructible(_Tp, _Up)>;
+
       // Constraint for a non-explicit constructor.
       // True iff each Ti in _Types... can be constructed from Ui in _UTypes...
       // and every Ui is implicitly convertible to Ti.
diff --git a/libstdc++-v3/testsuite/20_util/tuple/cons/96592.cc b/libstdc++-v3/testsuite/20_util/tuple/cons/96592.cc
new file mode 100644
index 0000000..326ab0e
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/tuple/cons/96592.cc
@@ -0,0 +1,58 @@
+// 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 } }
+
+#include <tuple>
+
+// PR 96592 comment 0
+
+template <typename SessionT>
+struct SomeQuery {
+    SessionT& session_;
+    SomeQuery(SessionT& session) : session_(session) {}
+};
+
+template <typename SessionT>
+struct Handler {
+    std::tuple<SomeQuery<SessionT>> queries_;
+    Handler(SessionT& session) : queries_(session) {}
+};
+
+struct Session {
+    Handler<Session> handler_;
+    Session() : handler_{*this} {}
+};
+
+Session session;
+
+// PR 96592 comment 1
+template <typename T>
+class DependsOnT
+{
+public:
+    DependsOnT(T&) {}
+};
+
+class Test
+{
+public:
+    Test() : test_{*this} {}
+
+private:
+    std::tuple<DependsOnT<Test>> test_;
+};