Commit 93bed40a authored by Committed by Marge Bot
Use injectiveVarsOfType to catch dodgy type family instance binders (#17008)
Previously, we detected dodgy type family instances binders by expanding type synonyms (via `exactTyCoVarsOfType`) and looking for type variables on the RHS that weren't mentioned on the (expanded) LHS. But this doesn't account for type families (like the example in #17008), so we instead use `injectiveVarsOfType` to only count LHS type variables that are in injective positions. That way, the `a` in `type instance F (x :: T a) = a` will not count if `T` is a type synonym _or_ a type family. Along the way, I moved `exactTyCoVarsOfType` to `TyCoFVs` to live alongside its sibling functions that also compute free variables. Fixes #17008.
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