Covariant means that 'this' can substitute for 't', i.e. a pure function is a match for an impure type.
type 'this' is covariant with
if not null, store STCxxxx which would make it covariant
true if extern(C++) function types should follow C++ covariant rules
An enum value of either Covariant.yes or a reason it's not covariant.
See Implementation
Covariant means that 'this' can substitute for 't', i.e. a pure function is a match for an impure type.