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There are several changes here.
* TcInteract has gotten too big, so I moved all the class-instance
matching out of TcInteract into a new module ClsInst. It parallels
the FamInst module.
The main export of ClsInst is matchGlobalInst.
This now works in TcM not TcS.
* A big reason to make matchGlobalInst work in TcM is that we can
then use it from TcValidity.checkSimplifiableClassConstraint.
That extends checkSimplifiableClassConstraint to work uniformly
for built-in instances, which means that we now get a warning
if we have givens (Typeable x, KnownNat n); see Trac #15322.
* This change also made me refactor LookupInstResult, in particular
by adding the InstanceWhat field. I also changed the name of the
type to ClsInstResult.
Then instead of matchGlobalInst reporting a staging error (which is
inappropriate for the call from TcValidity), we can do so in
TcInteract.checkInstanceOK.
* In TcValidity, we now check quantified constraints for termination.
For example, this signature should be rejected:
f :: (forall a. Eq (m a) => Eq (m a)) => blah
as discussed in Trac #15316. The main change here is that
TcValidity.check_pred_help now uses classifyPredType, and has a
case for ForAllPred which it didn't before.
This had knock-on refactoring effects in TcValidity.
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