[PATCH][Hashtable 5/6] Remove H1/H2 template parameters

[Jonathan Wakely]

On 26/08/20 16:55 +0100, Jonathan Wakely wrote:
>On 26/08/20 16:30 +0100, Jonathan Wakely wrote:
>>I'm seeing new FAILures with this:
>>
>>FAIL: 20_util/function_objects/searchers.cc (test for excess errors)
>>UNRESOLVED: 20_util/function_objects/searchers.cc compilation failed to produce executable
>>FAIL: experimental/functional/searchers.cc (test for excess errors)
>>UNRESOLVED: experimental/functional/searchers.cc compilation failed to produce executable
>>
>>It looks like what you committed is not what you sent for review. The
>>patch sent for review has:
>>
>>  /// Specialization: hash function and range-hashing function, no
>>  /// caching of hash codes.
>>  /// Provides typedef and accessor required by C++ 11.
>>  template<typename _Key, typename _Value, typename _ExtractKey,
>>-          typename _H1, typename _H2>
>>-    struct _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2,
>>-                          _Default_ranged_hash, false>
>>+          typename _Hash, typename _RangeHash, typename _Unused>
>>+    struct _Hash_code_base<_Key, _Value, _ExtractKey, _Hash, _RangeHash,
>>+                          _Unused, false>
>>    : private _Hashtable_ebo_helper<0, _ExtractKey>,
>>-      private _Hashtable_ebo_helper<1, _H1>,
>>-      private _Hashtable_ebo_helper<2, _H2>
>>+      private _Hashtable_ebo_helper<1, _Hash>
>>    {
>>
>>
>>But what you committed has:
>>
>>  /// Specialization: hash function and range-hashing function, no
>>  /// caching of hash codes.
>>  /// Provides typedef and accessor required by C++ 11.
>>  template<typename _Key, typename _Value, typename _ExtractKey,
>>-          typename _H1, typename _H2>
>>-    struct _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2,
>>-                          _Default_ranged_hash, false>
>>-    : private _Hashtable_ebo_helper<0, _ExtractKey>,
>>-      private _Hashtable_ebo_helper<1, _H1>,
>>-      private _Hashtable_ebo_helper<2, _H2>
>>+          typename _Hash, typename _RangeHash, typename _Unused>
>>+    struct _Hash_code_base<_Key, _Value, _ExtractKey, _Hash, _RangeHash,
>>+                          _Unused, false>
>>+    : private _Hashtable_ebo_helper<0, _Hash>
>>    {
>>
>>
>>Note that you've changed the type of the base class from:
>>
>>+      private _Hashtable_ebo_helper<1, _Hash>
>>
>>to
>>
>>+      private _Hashtable_ebo_helper<0, _Hash>
>>
>>This causes an ambiguity:
>>
>>/home/jwakely/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/hashtable_policy.h:1706: error: 'std::__detail::_Hashtable_ebo_helper<0, test03()::<unnamed struct>, true>' is an ambiguous base of 'std::__detail::_Hashtable_base<char, std::pair<const char, long int>, std::__detail::_Select1st, test03()::<unnamed struct>, test03()::<unnamed struct>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Hashtable_traits<true, false, true> >'
>>
>>However, what I don't understand is why we are storing that _Hash type
>>more than once as a base class. That seems wrong (but not something we
>>can change without ABI impact).
>
>Ah, we're not storing it more than once.
>
>The problem is:
>
>  template<typename _Key, typename _Value,
>	   typename _ExtractKey, typename _Equal,
>	   typename _H1, typename _H2, typename _Hash, typename _Traits>
>  struct _Hashtable_base
>  : public _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2, _Hash,
>			   _Traits::__hash_cached::value>,
>    private _Hashtable_ebo_helper<0, _Equal>
>
>This has a base of _Hashtable_ebo_helper<0, _Equal> so it used to
>have these bases:
>
>_Hashtable_ebo_helper<0, _ExtractKey>
>_Hashtable_ebo_helper<1, _Hash>
>_Hashtable_ebo_helper<2, _RangeHash>
>_Hashtable_ebo_helper<0, _Equal>
>
>but after your change it has these bases:
>
>_Hashtable_ebo_helper<0, _Hash>
>_Hashtable_ebo_helper<0, _Equal>
>
>In the case
>where _Equal and _Hash are the same type (which is what I was testing
>in the test that fail, because I'm sneaky) that means:
>
>_Hashtable_ebo_helper<0, T>
>_Hashtable_ebo_helper<0, T>
>
>which is obviously ambiguous.
>
>I think the _hash_code_base should still use the index 1 for its base
>class, i.e. _Hashtable_ebo_helper<1, _Hash>. That way we have these:
>
>_Hashtable_ebo_helper<1, _Hash>
>_Hashtable_ebo_helper<0, _Equal>
>
>which works even if they're the same types.

Here's a minimal test:

#include <unordered_map>

struct Evil : std::hash<int>, std::equal_to<int>
{
};

int main()
{
   std::unordered_map<int, int, Evil, Evil> h;
   h.key_eq();
}

This fails on current trunk.