The unordered associative containers defined in [unord] use specializations of the class template hash ([functional.syn]) as the default hash function.
Each specialization of hash is either enabled or disabled, as described below. [ Note: Enabled specializations meet the requirements of Hash, and disabled specializations do not. — end note ] Each header that declares the template hash provides enabled specializations of hash for nullptr_t and all cv-unqualified arithmetic, enumeration, and pointer types. For any type Key for which neither the library nor the user provides an explicit or partial specialization of the class template hash, hash<Key> is disabled.
If the library provides an explicit or partial specialization of hash<Key>, that specialization is enabled except as noted otherwise, and its member functions are noexcept except as noted otherwise.
If H is a disabled specialization of hash, these values are false: is_default_constructible_v<H>, is_copy_constructible_v<H>, is_move_constructible_v<H>, is_copy_assignable_v<H>, and is_move_assignable_v<H>. Disabled specializations of hash are not function object types. [ Note: This means that the specialization of hash exists, but any attempts to use it as a Hash will be ill-formed. — end note ]
An enabled specialization hash<Key> will:
satisfy the Hash requirements, with Key as the function call argument type, the DefaultConstructible requirements, the CopyAssignable requirements,
be swappable for lvalues,
satisfy the requirement that if k1 == k2 is true, h(k1) == h(k2) is also true, where h is an object of type hash<Key> and k1 and k2 are objects of type Key;
satisfy the requirement that the expression h(k), where h is an object of type hash<Key> and k is an object of type Key, shall not throw an exception unless hash<Key> is a user-defined specialization that depends on at least one user-defined type.