Clarify the scoping of existentials for pattern synonym signatures

What are scoping rules for type variables in pattern synonym signatures?

We said that existentials variables are scoped as follows:

1. Existentials scope over only the provided constraints and the

arguments, not over the result type.

type Disguised a b = b -> RP a                                                 
pattern Q :: () => Eq b => Disguised a b

However, Simon argues that Q should be accepted as if we expand the type synonym then the existentially quantified b is no longer in the result type.

Richard disagrees, he considers the result type to be Disguised a b and so b is in the result type and not in-scope. Therefore he wishes to reject this example.

The purpose of this ticket is to decide the fate of Q.

This kind of problem is related to #12108 (closed), #12179