Richard Eisenberg authored Dec 05, 2019 and Marge Bot committed Feb 11, 2020

Previously, we would accidentally make constraints like
forall a. C a => forall b. D b => E a b c as we traversed
superclasses. No longer!

This patch also expands Note [Eagerly expand given superclasses]
to work over quantified constraints; necessary for T16502b.

Close #17202 and #16502.

test cases: typecheck/should_compile/T{17202,16502{,b}}