How to use `invoke` with a functor

This is maybe a stupid question, but I couldn’t find the answer in the documentation. I need it mainly for debugging, but a minimal example would be

abstract type A end
(::A)() = "A"
struct B <: A end
(::B)() = "B"
b = B()

How to call the first method from b ? I.e. is there any syntax for the call function to use with invoke?