[Pitch 2] Light-weight same-type requirement syntax

[hborla (Holly Borla)]

@hborla

As a result of the feedback here, I'm putting together the set of ideas for a general opaque result type constraint syntax that I know of to start a new discussion to get all of your insight and brainstorm together.

As promised:

Generalized opaque and existential type constraints

Constraints on opaque and existential types are currently limited to protocol and protocol composition requirements on the underlying type. More advanced requirements, such as constraining an associated type, are currently not supported in the language. [Pitch 2] Light-weight same-type requirement syntax proposes to add a limited form of same-type requirement on "primary" associated types, but this feature is not a replacement for a more general syntax. This post outlines a few ideas for expres…