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

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

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