Type family in instance signature confuses GHC

This works:

class F a where
  type T a
  u :: T a -> a -> a

newtype W a = W a

instance F (W a) where
  type T (W a) = T a
  u = undefined 

instance F () where
  type T () = T (W ())
  u _ = undefined 

Remove one argument to u and you get

    • Reduction stack overflow; size = 201
      When simplifying the following type: T (W ())
      Use -freduction-depth=0 to disable this check
      (any upper bound you could choose might fail unpredictably with
       minor updates to GHC, so disabling the check is recommended if
       you're sure that type checking should terminate)
    • In the expression: undefined
      In an equation for ‘u’: u = undefined
      In the instance declaration for ‘F ()’

It is also not accepted to

  u :: T (W ()) -> () -> ()
  u _ = id @()

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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