A year ago I gave up on trying to rigorously separate TyVars
from TcTyVars, and instead allowed TyVars to appear rather more
freely in types examined by the constraint solver:

   commit 18d0bdd3
   Author: Simon Peyton Jones <simonpj@microsoft.com>
   Date:   Wed Nov 23 16:00:00 2016 +0000

   Allow TyVars in TcTypes

See Note [TcTyVars in the typechecker] in TcType.

However, TcFlatten.flatten_tyvar1 turned out to treat
a TyVar specially, and implicitly assumed that it could
not have an equality constraint in the inert set.  Wrong!

This caused Trac #14450.  Fortunately it is easily fixed,
by deleting code.