Generalized opaque and existential type constraints

[John_McCall]

@bjhomer

Okay, I was just thinking in terms of function declarations, but this helped me understand that there's a larger picture here. If I understand correctly, there's a desire to build a syntax for declaring constraints on opaque and existential types throughout the language . For the sake of making sure I understand that desire, I'd like to sketch out a few scenarios to make sure I understand.

In addition to Holly's post, I tried to lay out the larger picture here.