Ryan Scott authored Jul 26, 2019 and Marge Bot committed Oct 07, 2019

Among other uses, `coreFlattenTyFamApp` is used by Core Lint as a
part of its check to ensure that each type family axiom reduces
according to the way it is defined in the source code. Unfortunately,
the logic that `coreFlattenTyFamApp` uses to flatten type family
applications disagreed with the logic in `TcFlatten`, which caused
it to spuriously complain this program:

```hs
type family Param :: Type -> Type

type family LookupParam (a :: Type) :: Type where
  LookupParam (f Char) = Bool
  LookupParam x        = Int

foo :: LookupParam (Param ())
foo = 42
```

This is because `coreFlattenTyFamApp` tries to flatten the `Param ()`
in `LookupParam (Param ())` to `alpha` (where `alpha` is a flattening
skolem), and GHC is unable to conclude that `alpha` is apart from
`f Char`. This patch spruces up `coreFlattenTyFamApp` so that it
instead flattens `Param ()` to `alpha ()`, which GHC _can_ know for
sure is apart from `f Char`. See
`Note [Flatten], wrinkle 3` in `FamInstEnv`.