Let me know if I need to reference a specific paper or any other 
Standard reference here. Maybe P1690R1 I used here ?

I tried to allow the same partition trick you can have on ordered 
containers (see Partition in tests) even if here elements are not 
ordered so I aren't sure there can be any usage of it.

     libstdc++: Add unordered containers heterogeneous lookup

     Add unordered containers heterogeneous lookup member functions 
find, count, contains and
     equal_range in C++20. Those members are considered for overload 
resolution only if hash and
     equal functors used to instantiate the container have a nested 
is_transparent type.

     libstdc++-v3/ChangeLog:

             * include/bits/stl_tree.h
             (__has_is_transparent, __has_is_transparent_t): Move...
             * include/bits/stl_function.h: ...here.
             * include/bits/hashtable_policy.h 
(_Hash_code_base<>::_M_hash_code):
             Use template key type.
             (_Hashtable_base<>::_M_equals): Likewise.
             * include/bits/hashtable.h (_Hashtable<>::_M_find_tr, 
_Hashtable<>::_M_count_tr,
             _Hashtable<>::_M_equal_range_tr): New member function 
templates to perform
             heterogeneous lookup.
             (_Hashtable<>::_M_find_before_node): Use template key type.
             (_Hashtable<>::_M_find_node): Likewise.
             * include/bits/unordered_map.h (unordered_map::find<>, 
unordered_map::count<>,
             unordered_map::contains<>, unordered_map::equal_range<>): 
New member function
             templates to perform heterogeneous lookup.
             (unordered_multimap::find<>, unordered_multimap::count<>,
             unordered_multimap::contains<>, 
unordered_multimap::equal_range<>): Likewise.
             * include/bits/unordered_set.h (unordered_set::find<>, 
unordered_set::count<>,
             unordered_set::contains<>, unordered_set::equal_range<>): 
Likewise.
             (unordered_multiset::find<>, unordered_multiset::count<>,
             unordered_multiset::contains<>, 
unordered_multiset::equal_range<>): Likewise.
             * include/debug/unordered_map
             (unordered_map::find<>, unordered_map::equal_range<>): 
Likewise.
             (unordered_multimap::find<>, 
unordered_multimap::equal_range<>): Likewise.
             * include/debug/unordered_set
             (unordered_set::find<>, unordered_set::equal_range<>): 
Likewise.
             (unordered_multiset::find<>, 
unordered_multiset::equal_range<>): Likewise.
             * testsuite/23_containers/unordered_map/operations/1.cc: 
New test.
             * 
testsuite/23_containers/unordered_multimap/operations/1.cc: New test.
             * 
testsuite/23_containers/unordered_multiset/operations/1.cc: New test.
             * testsuite/23_containers/unordered_set/operations/1.cc: 
New test.

Tested under Linux x86_64 normal and debug modes.

François