Ryan Scott authored Oct 03, 2019 and Marge Bot committed Oct 23, 2019

`TcSplice` was not properly handling oversaturated data family
instances, such as the example in #17296, as it dropped arguments due
to carelessly zipping data family instance arguments with
`tyConTyVars`. For data families, the number of `tyConTyVars` can
sometimes be less than the number of arguments it can accept in a
data family instance due to the fact that data family instances can
be oversaturated.

To account for this, `TcSplice.mkIsPolyTvs` has now been renamed to
`tyConArgsPolyKinded` and now factors in `tyConResKind` in addition
to `tyConTyVars`. I've also added
`Note [Reified instances and explicit kind signatures]` which
explains the various subtleties in play here.

Fixes #17296.