Note [The QLInstVar TcLevel]

QuickLook instantiation variables are identified by having a TcLevel
of QLInstVar.  See Note [Quick Look overview] in GHC.Tc.Gen.App.

The QLInstVar level behaves like infinity: it is greater than any
other TcLevel.  See `strictlyDeeperThan` and friends in this module.
That ensures that we never unify an ordinary unification variable
with a QL instantiation variable, e.g.
      alpha[tau:3] := Maybe beta[tau:qlinstvar]
(This is an immediate consequence of our general rule that we never
unify a variable with a type mentioning deeper variables; the skolem
escape check.)

QL instantation variables are eventually turned into ordinary unificaiton
variables; see (QL3) in Note [Quick Look overview].