20 General utilities library [utilities]

20.21 Stacktrace [stacktrace]

20.21.4 Class template basic_­stacktrace [stacktrace.basic]

20.21.4.7 Hash support [stacktrace.basic.hash]

template<> struct hash<stacktrace_entry>; template<class Allocator> struct hash<basic_stacktrace<Allocator>>;
The specializations are enabled ([unord.hash]).