T4099.hs:11:14:
    Couldn't match type `T b' with `T a0'
    NB: `T' is a type function, and may not be injective
    In the first argument of `foo', namely `x'
    In the expression: foo x
    In an equation for `bar1': bar1 x = foo x

T4099.hs:14:14:
    Couldn't match type `T a1' with `Maybe b'
    In the first argument of `foo', namely `x'
    In the expression: foo x
    In an equation for `bar2': bar2 x = foo x