Re: [v3 PATCH] PR libstdc++/60970, implement LWG 2148

[Jonathan Wakely <jwakely at redhat dot com>]

On 25/07/15 22:01 +0300, Ville Voutilainen wrote:
> 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).

Committed with a minor change to put is_enum<_Tp>::value as a default
template argument of __hash_enum and add a comment.

commit 7d23fa744a6eb0ee4d0df9f044b72cb172336915
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jul 29 11:09:52 2015 +0100

    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.

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));
+}