T6054.hs:7:14:
    No instance for (Bar () '() a0)
      arising from an expression type signature
    The type variable `a0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    In the first argument of `print', namely
      `(Proxy :: Bar () a => Proxy a)'
    In the expression: print (Proxy :: Bar () a => Proxy a)
    In an equation for `foo':
        foo = print (Proxy :: Bar () a => Proxy a)