PR libstdc++/56267
	* include/bits/hashtable.h (__cache_default): Check if hash function
	is copy assignable.
	* testsuite/23_containers/unordered_set/56267.cc: New.
	* testsuite/23_containers/unordered_set/instantiation_neg.cc: Adjust
	dg-error line number.
	* testsuite/23_containers/unordered_set/
	not_default_constructible_hash_neg.cc: Likewise.

From-SVN: r195936