[PATCH] Implement LWG 2686, hash<error_condition>

[Jonathan Wakely]

On 07/05/19 10:37 +0100, Jonathan Wakely wrote:
>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

Is it definitely the new 19_diagnostics/error_condition/hash.cc test
that's giving this error?

I adjusted the pre-existing 27_io/filesystem/operations/absolute.cc
test in r270874, which seems a more likely culprit, but that already
has dg-require-filesystem-ts.