TyFamUndec.hs:6:15:
    Variable ‘b’ occurs more often than in the instance head
      in the type family application: T (b, b)
    (Use UndecidableInstances to permit this)
    In the type instance declaration for ‘T’

TyFamUndec.hs:7:15:
    Application is no smaller than the instance head
      in the type family application: T (a, Maybe b)
    (Use UndecidableInstances to permit this)
    In the type instance declaration for ‘T’

TyFamUndec.hs:8:15:
    Nested type family application
      in the type family application: T (a, T b)
    (Use UndecidableInstances to permit this)
    In the type instance declaration for ‘T’