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Quick Look does not zonk return types

Test case impredicative/T17332:

data Dict c where
  MkDict :: c => Dict c

aux :: Dict (forall a. a)
aux = MkDict

The constraint c ends up in the solver as an IrredPred, which reveals the polytype forall a. a. But zonking to reveal a polytype should never happen outside of Quick Look, as Note [Quick Look] in GHC.Tc.Gen.App tells us.

A quick look didn't reveal exactly where to put the zonk, and I don't want to add yet another wrinkle to !4149 (closed). But when this bug is fixed, the ForAllPred case in canIrred should never happen.

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