Fix floating of equalities

This rather subtle patch fixes Trac #14584.  The problem was
that we'd allowed a coercion, bound in a nested scope, to escape
into an outer scope.

The main changes are

* TcSimplify.floatEqualities takes more care when floating
  equalities to make sure we don't float one out that mentions
  a locally-bound coercion.
  See Note [What prevents a constraint from floating]

* TcSimplify.emitResidualConstraints (which emits the residual
  constraints in simplifyInfer) now avoids burying the constraints
  for escaping CoVars inside the implication constraint.

* Since I had do to this stuff with CoVars, I moved the
  fancy footwork about not quantifying over CoVars from
  TcMType.quantifyTyVars to its caller
  TcSimplify.decideQuantifiedTyVars.  I think its other
  callers don't need to worry about all this CoVar stuff.

This turned out to be surprisigly tricky, and took me a solid
day to get right.  I think the result is reasonably neat, though,
and well documented with Notes.

0 files changed, 0 insertions, 0 deletions