T4099.hs:11:14:
    Couldn't match expected type `T a0' with actual type `T b'
    NB: `T' is a type function, and may not be injective
    The type variable `a0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    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 expected type `T a1' with actual type `Maybe b'
    The type variable `a1' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    In the first argument of `foo', namely `x'
    In the expression: foo x
    In an equation for `bar2': bar2 x = foo x