Note [Shortcut solving: incoherence]

This optimization relies on coherence of dictionaries to be correct. When we
cannot assume coherence because of IncoherentInstances then this optimization
can change the behavior of the user's code.

The following four modules produce a program whose output would change depending
on whether we apply this optimization when IncoherentInstances is in effect:

=========
    {-# LANGUAGE MultiParamTypeClasses #