Oh, now I see one of the rabbit holes. It took explicitly writing that above-quoted assumption to notice what one of the challenges to my logic is here: even in my expected treatment of this, the compiler doesn’t necessarily know that the type parameter of the FooPsiVisitor used in its first argument visitor is the definitive informer of what type TReturn is, does it?
D’oh.
(::dreaming:: It would be nice to have a facility to indicate that once such part of a signature is the definitive informer of a type parameter 
)