Tested on Linux-PPC64.
The proposed resolution of the issue doesn't really say whether our
regression test for PR libstdc++/52931 should remain valid. However,
it doesn't say that we shouldn't keep it valid, either. This approach
keeps it valid, but provides support for hashing enums. It took a while
to figure out suitable jiggery-pokery to make it so, but this approach
passes the testsuite without regressions. I considered an alternative
alias-template-based approach, but while that attempt would've worked
with our current front-end, it would not have worked on clang (and
it's thus far unclear whether it was intended to work by the language
rules).
2015-07-25 Ville Voutilainen <ville.voutilai...@gmail.com>
PR libstdc++/60970, implement LWG 2148, hash support for
enum types.
* include/bits/functional_hash.h
(__hash_enum): New.
(hash): Derive from __hash_enum.
* testsuite/20_util/hash/60970.cc: New.
diff --git a/libstdc++-v3/include/bits/functional_hash.h
b/libstdc++-v3/include/bits/functional_hash.h
index d94843f..f81864f 100644
--- a/libstdc++-v3/include/bits/functional_hash.h
+++ b/libstdc++-v3/include/bits/functional_hash.h
@@ -57,6 +57,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp>
struct hash;
+ // Helper struct for SFINAE-poisoning non-enum types.
+ template<typename _Tp, bool>
+ struct __hash_enum
+ {
+ private:
+ __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, is_enum<_Tp>::value>
+ { };
+
/// 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));
+}
