Heterogenous lookup for unordered containers is a new feature in C++20, and thus is guarded by /std:c++latest. The paper accepted at the Belfast C++ meeting, P1690R1, changed the interface to require you to specify a matching equality comparison predicate for your hasher. Thus, for:

std::unordered_map<std::string, int, string_hash> map;

you need to say:

std::unordered_map<std::string, int, string_hash, std::equal_to<>> map;

As this change was directed by the C++ standardization committee, the change in product behavior is by design and not a bug.

Have a great day!

Billy O’Neal
Visual C++ Libraries