The shortcut solver for type classes (Trac #12791) was eagerly
solving a constaint from an OVERLAPPABLE instance. It happened
to be the only one in scope, so it was unique, but since it's
specfically flagged as overlappable it's really a bad idea to
solve using it, rather than using the Given dictionary.

This led to Trac #14434, a nasty and hard to identify bug.