This re-applies
 commit cc6be3a2
  Author: Ben Gamari <bgamari.foss@gmail.com>
  Date:   Tue Sep 19 18:57:38 2017 -0400

    Typeable: Allow App to match arrow types

which was reverted because of Trac #14270.  Now the latter is
fixed we can re-apply it.

The original ticket was Trac #14236