Ryan Scott authored Apr 04, 2021 and Marge Bot committed Apr 08, 2021

Previously, associated type family instances would incorrectly claim to
implicitly quantify over type variables bound by the instance head in the
`HsOuterImplicit`s that `rnFamEqn` returned. This is fixed by using
`filterInScopeM` to filter out any type variables that the instance head
binds.

Fixes #19649.