Commit a19c7d17 authored by Committed by Marge Bot
Reify oversaturated data family instances correctly (#17296)
`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.
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