[v3 PATCH] LWG 2510, make the default constructors of library tag types explicit.

[Ville Voutilainen <ville.voutilainen@gmail.com>]

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Tested on Linux-X64.

2015-11-10  Ville Voutilainen  <ville.voutilainen@gmail.com>

    LWG 2510, make the default constructors of library tag types
    explicit.
    * include/bits/mutex.h (defer_lock_t, try_lock_t,
    adopt_lock_t): Add an explicit default constructor.
    * include/bits/stl_pair.h (piecewise_construct_t): Likewise.
    * include/bits/uses_allocator.h (allocator_arg_t): Likewise.
    * libsupc++/new (nothrow_t): Likewise.
    * testsuite/17_intro/tag_type_explicit_ctor.cc: New.

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diff --git a/libstdc++-v3/include/bits/mutex.h b/libstdc++-v3/include/bits/mutex.h
index 43f5b0b..dd27989 100644
--- a/libstdc++-v3/include/bits/mutex.h
+++ b/libstdc++-v3/include/bits/mutex.h
@@ -129,14 +129,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // _GLIBCXX_HAS_GTHREADS
 
   /// Do not acquire ownership of the mutex.
-  struct defer_lock_t { };
+  struct defer_lock_t { explicit defer_lock_t() = default; };
 
   /// Try to acquire ownership of the mutex without blocking.
-  struct try_to_lock_t { };
+  struct try_to_lock_t { explicit try_to_lock_t() = default; };
 
   /// Assume the calling thread has already obtained mutex ownership
   /// and manage it.
-  struct adopt_lock_t { };
+  struct adopt_lock_t { explicit adopt_lock_t() = default; };
 
   constexpr defer_lock_t	defer_lock { };
   constexpr try_to_lock_t	try_to_lock { };
diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
index dfcd357..d6f6b86 100644
--- a/libstdc++-v3/include/bits/stl_pair.h
+++ b/libstdc++-v3/include/bits/stl_pair.h
@@ -73,7 +73,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if __cplusplus >= 201103L
   /// piecewise_construct_t
-  struct piecewise_construct_t { };
+  struct piecewise_construct_t { explicit piecewise_construct_t() = default; };
 
   /// piecewise_construct
   constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
diff --git a/libstdc++-v3/include/bits/uses_allocator.h b/libstdc++-v3/include/bits/uses_allocator.h
index f9ea7d6..a0f084d 100644
--- a/libstdc++-v3/include/bits/uses_allocator.h
+++ b/libstdc++-v3/include/bits/uses_allocator.h
@@ -36,7 +36,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// [allocator.tag]
-  struct allocator_arg_t { };
+  struct allocator_arg_t { explicit allocator_arg_t() = default; };
 
   constexpr allocator_arg_t allocator_arg = allocator_arg_t();
 
diff --git a/libstdc++-v3/libsupc++/new b/libstdc++-v3/libsupc++/new
index 0f6a05a..8621f73 100644
--- a/libstdc++-v3/libsupc++/new
+++ b/libstdc++-v3/libsupc++/new
@@ -79,7 +79,12 @@ namespace std
   };
 #endif
 
-  struct nothrow_t { };
+  struct nothrow_t
+  {
+#if __cplusplus >= 201103L
+    explicit nothrow_t() = default;
+#endif
+  };
 
   extern const nothrow_t nothrow;
 
diff --git a/libstdc++-v3/testsuite/17_intro/tag_type_explicit_ctor.cc b/libstdc++-v3/testsuite/17_intro/tag_type_explicit_ctor.cc
new file mode 100644
index 0000000..4b9d217
--- /dev/null
+++ b/libstdc++-v3/testsuite/17_intro/tag_type_explicit_ctor.cc
@@ -0,0 +1,60 @@
+// { dg-do compile }
+// { dg-options "-std=gnu++11" }
+
+// Copyright (C) 2015 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 <new>
+#include <utility>
+#include <memory>
+#include <mutex>
+
+void f1(std::nothrow_t);
+void f2(std::piecewise_construct_t);
+void f3(std::allocator_arg_t);
+void f4(std::defer_lock_t);
+void f5(std::try_to_lock_t);
+void f6(std::adopt_lock_t);
+
+
+int main()
+{
+  std::nothrow_t v1;
+  std::piecewise_construct_t v2;
+  std::allocator_arg_t v3;
+  std::defer_lock_t v4;
+  std::try_to_lock_t v5;
+  std::try_to_lock_t v6;
+  std::nothrow_t v7 = {}; // { dg-error "explicit" }
+  std::piecewise_construct_t v8 = {}; // { dg-error "explicit" }
+  std::allocator_arg_t v9 = {}; // { dg-error "explicit" }
+  std::defer_lock_t v10 = {}; // { dg-error "explicit" }
+  std::try_to_lock_t v11 = {}; // { dg-error "explicit" }
+  std::try_to_lock_t v12 = {}; // { dg-error "explicit" }
+  f1(std::nothrow_t{});
+  f2(std::piecewise_construct_t{});
+  f3(std::allocator_arg_t{});
+  f4(std::defer_lock_t{});
+  f5(std::try_to_lock_t{});
+  f6(std::adopt_lock_t{});
+  f1({}); // { dg-error "explicit" }
+  f2({}); // { dg-error "explicit" }
+  f3({}); // { dg-error "explicit" }
+  f4({}); // { dg-error "explicit" }
+  f5({}); // { dg-error "explicit" }
+  f6({}); // { dg-error "explicit" }
+}