https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105907
Bug ID: 105907
Summary: unordered containers should cache result of
std::hash<std::pmr::string>
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: ABI
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
The std::hash specializations for PMR strings do not have corresponding
specializations of the __is_fast_hash trait, which means they use the default
i.e. fast-hash == true. This means that unordered containers do not bother to
cache the hash code.
This is wrong, hashing PMR strings is exactly as slow as hashing non-PMR
strings. But changing it now would be an ABI break for e.g.
std::unordered_set<std::pmr::string> because it would add a hash code member to
each node.
Similarly, adding specializations of __is_fast_hash corresponding to the
changes for https://wg21.link/lwg3705 ("Hashability shouldn't depend on
basic_string's allocator") will be an ABI break for e.g.
std::unordered_set<std::basic_string<char, std::char_traits<char>, MyAlloc>>
We really should have just defined __is_fast_hash to be true for all
specializations of basic_string, instead of only the ones that have a
predefined std::hash specialization.
