A specialization of class template pmr::polymorphic_allocator
if its template argument is a cv
-unqualified object type.
Constructed with different memory resources,
different instances of the same specialization of pmr::polymorphic_allocator
can exhibit entirely different allocation behavior.
This runtime polymorphism allows objects that use polymorphic_allocator
to behave as if they used different allocator types at run time
even though they use the same static allocator type.