re PR libstdc++/60970 (Support std::hash with enum types (LWG 2148))

2015-07-29  Ville Voutilainen  <ville.voutilainen@gmail.com>

	PR libstdc++/60970
	* include/bits/functional_hash.h (__hash_enum): New.
	(hash): Derive from __hash_enum.
	* testsuite/20_util/hash/60970.cc: New.

From-SVN: r226347

3 files changed, 71 insertions, 0 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 4447b3d..f0f8c1a 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2015-07-29  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+	PR libstdc++/60970
+	* include/bits/functional_hash.h (__hash_enum): New.
+	(hash): Derive from __hash_enum.
+	* testsuite/20_util/hash/60970.cc: New.
+
 2015-07-29  Tim Shen  <timshen@google.com>
 
 	PR libstdc++/67015
diff --git a/libstdc++-v3/include/bits/functional_hash.h b/libstdc++-v3/include/bits/functional_hash.h
index 3c962fc..88937bd 100644
--- a/libstdc++-v3/include/bits/functional_hash.h
+++ b/libstdc++-v3/include/bits/functional_hash.h
@@ -57,6 +57,34 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp>
     struct hash;
 
+  // Helper struct for SFINAE-poisoning non-enum types.
+  template<typename _Tp, bool = is_enum<_Tp>::value>
+    struct __hash_enum
+    {
+    private:
+      // Private rather than deleted to be non-trivially-copyable.
+      __hash_enum(__hash_enum&&);
+      ~__hash_enum();
+    };
+
+  // Helper struct for hash with enum types.
+  template<typename _Tp>
+    struct __hash_enum<_Tp, true> : public __hash_base<size_t, _Tp>
+    {
+      size_t
+      operator()(_Tp __val) const noexcept
+      {
+       using __type = typename underlying_type<_Tp>::type;
+       return hash<__type>{}(static_cast<__type>(__val));
+      }
+    };
+
+  /// Primary class template hash, usable for enum types only.
+  // Use with non-enum types still SFINAES.
+  template<typename _Tp>
+    struct hash : __hash_enum<_Tp>
+    { };
+
   /// Partial specializations for pointer types.
   template<typename _Tp>
     struct hash<_Tp*> : public __hash_base<size_t, _Tp*>
diff --git a/libstdc++-v3/testsuite/20_util/hash/60970.cc b/libstdc++-v3/testsuite/20_util/hash/60970.cc
new file mode 100644
index 0000000..ddc626f
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/hash/60970.cc
@@ -0,0 +1,36 @@
+// { dg-options "-std=gnu++11" }
+// { dg-do run }
+
+// 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 <functional>
+#include <testsuite_hooks.h>
+
+using namespace std;
+
+enum E1 : int {FIRST=1, SECOND=2};
+enum class E2 : int {THIRD=42, FOURTH=666};
+
+int main()
+{
+  VERIFY(hash<int>{}(1) == hash<E1>{}(FIRST));
+  VERIFY(hash<int>{}(2) == hash<E1>{}(SECOND));
+  VERIFY(hash<int>{}(42) == hash<E2>{}(E2::THIRD));
+  VERIFY(hash<int>{}(666) == hash<E2>{}(E2::FOURTH));
+}