[PATCH] Implement LWG 2686, hash<error_condition>

[Jonathan Wakely]

On 07/05/19 11:05 +0200, Christophe Lyon wrote:
>On Sat, 4 May 2019 at 16:36, Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> On 03/05/19 23:42 +0100, Jonathan Wakely wrote:
>> >On 23/03/17 17:49 +0000, Jonathan Wakely wrote:
>> >>On 12/03/17 13:16 +0100, Daniel Krügler wrote:
>> >>>The following is an *untested* patch suggestion, please verify.
>> >>>
>> >>>Notes: My interpretation is that hash<error_condition> should be
>> >>>defined outside of the _GLIBCXX_COMPATIBILITY_CXX0X block, please
>> >>>double-check that course of action.
>> >>
>> >>That's right.
>> >>
>> >>>I noticed that the preexisting hash<error_code> did directly refer to
>> >>>the private members of error_code albeit those have public access
>> >>>functions. For consistency I mimicked that existing style when
>> >>>implementing hash<error_condition>.
>> >>
>> >>I see no reason for that, so I've removed the friend declaration and
>> >>used the public member functions.
>> >
>> >I'm going to do the same for hash<error_code> too. It can also use the
>> >public members instead of being a friend.
>> >
>> >
>> >>Although this is a DR, I'm treating it as a new C++17 feature, so I've
>> >>adjusted the patch to only add the new specialization for C++17 mode.
>> >>We're too close to the GCC 7 release to be adding new things to the
>> >>default mode, even minor things like this. After GCC 7 is released we
>> >>can revisit it and decide if we want to enable it for all modes.
>> >
>> >We never revisited that, and it's still only enabled for C++17 and up.
>> >I guess that's OK, but we could enabled it for C++11 and 14 on trunk
>> >if we want. Anybody care enough to argue for that?
>> >
>> >>Here's what I've tested and will be committing.
>> >>
>> >>
>> >
>> >>commit 90ca0fd91f5c65af370beb20af06bdca257aaf63
>> >>Author: Jonathan Wakely <jwakely@redhat.com>
>> >>Date:   Thu Mar 23 11:47:39 2017 +0000
>> >>
>> >>   Implement LWG 2686, std::hash<error_condition>, for C++17
>> >>   2017-03-23  Daniel Kruegler  <daniel.kruegler@gmail.com>
>> >>      Implement LWG 2686, Why is std::hash specialized for error_code,
>> >>      but not error_condition?
>> >>      * include/std/system_error (hash<error_condition>): Define for C++17.
>> >>      * testsuite/20_util/hash/operators/size_t.cc (hash<error_condition>):
>> >>      Instantiate test for error_condition.
>> >>      * testsuite/20_util/hash/requirements/explicit_instantiation.cc
>> >>      (hash<error_condition>): Instantiate hash<error_condition>.
>> >>
>> >>diff --git a/libstdc++-v3/include/std/system_error b/libstdc++-v3/include/std/system_error
>> >>index 6775a6e..ec7d25f 100644
>> >>--- a/libstdc++-v3/include/std/system_error
>> >>+++ b/libstdc++-v3/include/std/system_error
>> >>@@ -373,14 +373,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >>_GLIBCXX_END_NAMESPACE_VERSION
>> >>} // namespace
>> >>
>> >>-#ifndef _GLIBCXX_COMPATIBILITY_CXX0X
>> >>-
>> >>#include <bits/functional_hash.h>
>> >>
>> >>namespace std _GLIBCXX_VISIBILITY(default)
>> >>{
>> >>_GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >>
>> >>+#ifndef _GLIBCXX_COMPATIBILITY_CXX0X
>> >>  // DR 1182.
>> >>  /// std::hash specialization for error_code.
>> >>  template<>
>> >>@@ -394,12 +393,27 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> >>      return std::_Hash_impl::__hash_combine(__e._M_cat, __tmp);
>> >>      }
>> >>    };
>> >>+#endif // _GLIBCXX_COMPATIBILITY_CXX0X
>> >>+
>> >>+#if __cplusplus > 201402L
>> >>+  // DR 2686.
>> >>+  /// std::hash specialization for error_condition.
>> >>+  template<>
>> >>+    struct hash<error_condition>
>> >>+    : public __hash_base<size_t, error_condition>
>> >>+    {
>> >>+      size_t
>> >>+      operator()(const error_condition& __e) const noexcept
>> >>+      {
>> >>+     const size_t __tmp = std::_Hash_impl::hash(__e.value());
>> >>+     return std::_Hash_impl::__hash_combine(__e.category(), __tmp);
>> >
>> >When I changed this from using __e._M_cat (as in Daniel's patch) to
>> >__e.category() I introduced a bug, because the former is a pointer to
>> >the error_category (and error_category objects are unique and so can
>> >be identified by their address) and the latter is the object itself,
>> >so we hash the bytes of an abstract base class instead of hashing the
>> >pointer to it. Oops.
>> >
>> >Patch coming up to fix that.
>>
>> Here's the fix. Tested powerpc64le-linux, committed to trunk.
>>
>> I'll backport this to 7, 8 and 9 as well.
>>
>
>Hi Jonathan,
>
>Does the new test lack dg-require-filesystem-ts ?

It lacks it, because it doesn't use the filesystem library at all.

>I'm seeing link failures on arm-eabi (using newlib):
>Excess errors:
>/libstdc++-v3/src/c++17/fs_ops.cc:806: undefined reference to `chdir'
>/libstdc++-v3/src/c++17/fs_ops.cc:583: undefined reference to `mkdir'
>/libstdc++-v3/src/c++17/fs_ops.cc:1134: undefined reference to `chmod'
>/libstdc++-v3/src/c++17/../filesystem/ops-common.h:439: undefined
>reference to `chmod'
>/libstdc++-v3/src/c++17/fs_ops.cc:750: undefined reference to `pathconf'
>/libstdc++-v3/src/c++17/fs_ops.cc:769: undefined reference to `getcwd'
>
>Christophe