Re: [PATCH] Hashtable PR96088

[Jonathan Wakely <jwakely@redhat.com>]

On 24/05/21 11:31 +0200, François Dumont wrote:
>On 20/05/21 6:44 pm, Jonathan Wakely wrote:
>>On 06/05/21 22:03 +0200, François Dumont via Libstdc++ wrote:
>>>Hi
>>>
>>>    Considering your feedback on backtrace in debug mode is going 
>>>to take me some time so here is another one.
>>>
>>>    Compared to latest submission I've added a _Hash_arg_t partial 
>>>specialization for std::hash<>. It is not strictly necessary for 
>>>the moment but when we will eventually remove its nested 
>>>argument_type it will be. I also wonder if it is not easier to 
>>>handle for the compiler, not sure about that thought.
>>
>>The std::hash specializations in libstdc++ define argument_type, but
>>I'm already working on one that doesn't (forstd::stacktrace).
>>
>>And std::hash<acme::ProgramDefinedType> can be specialized by users,
>>and is not required to provide argument_type.
>>
>>So it's already not valid to assume that std::hash<T>::argument_type
>>exists.
>>
>>>@@ -850,9 +852,56 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>>    iterator
>>>    _M_emplace(const_iterator, false_type __uks, _Args&&... __args);
>>>
>>>+      template<typename _Kt, typename _Arg, typename _NodeGenerator>
>>>+    std::pair<iterator, bool>
>>>+    _M_insert_unique(_Kt&&, _Arg&&, const _NodeGenerator&);
>>>+
>>>+      // Detect nested argument_type.
>>>+      template<typename _Kt, typename _Ht, typename = __void_t<>>
>>>+    struct _Hash_arg_t
>>>+    { typedef _Kt argument_type; };
>>>+
>>>+      // std::hash
>>>+      template<typename _Kt, typename _Arg>
>>>+    struct _Hash_arg_t<_Kt, std::hash<_Arg>>
>>>+    { typedef _Arg argument_type; };
>>>+
>>>+      // Nested argument_type.
>>>+      template<typename _Kt, typename _Ht>
>>>+    struct _Hash_arg_t<_Kt, _Ht,
>>>+              __void_t<typename _Ht::argument_type>>
>>>+    { typedef typename _Ht::argument_type argument_type; };
>>>+
>>>+      // Function pointer.
>>>+      template<typename _Kt, typename _Arg>
>>>+    struct _Hash_arg_t<_Kt, std::size_t(*)(const _Arg&)>
>>>+    { typedef _Arg argument_type; };
>>>+
>>>+      template<typename _Kt,
>>>+           typename _ArgType
>>>+         = typename _Hash_arg_t<_Kt, _Hash>::argument_type>
>>>+    static typename conditional<
>>>+      __is_nothrow_convertible<_Kt, _ArgType>::value, _Kt&&, 
>>>key_type>::type
>>
>>Please use __conditional_t<...> here instead of
>>typename conditional<...>::type.
>
>I forgot to say in my new proposal that I didn't find the 
>__conditional_t. I can see a std::condition_t but only in C++14.
>
>Do you want to add a __conditional_t for C++11 ?

Doh, I forgot that's only in my fork not in the gcc.gnu.org repo.

No need to add it, using typename conditional<>::type is fine here.