Failure of improvement for type-family dependencies

Consider

{-# LANGUAGE TypeFamilyDependencies #-}

data D x

type family F t = s | s -> t
type instance F (D t) = D (F t)

f :: F s -> ()
f _ = ()

g :: D (F t) -> ()
g x = f x

which was presented in this email from Clinton Mead.

It yields

Tx.hs:14:9: error:
    • Couldn't match expected type ‘F s0’ with actual type ‘D (F t)’
      The type variable ‘s0’ is ambiguous

But it should work fine

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Related
Blocking
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Operating system
Architecture