Fix impredicativity (again)
This patch fixes Trac #12616. Dignosis. In TcUnify.tc_sub_type_ds we were going to some trouble to support co- and contra-variance even for impredicative types. With -XImpredicativeTYpes, this allowed a unification variable to be unified with a polytype (probably wrongly) and that caused later trouble in the constraint solver, where -XImpredicativeTypes was /not/ on. In effect, -XImpredicativeTypes can't be switched on locally. Why did we want ImpredicativeTypes locally? Because the program generated by GND for a higher-rank method involved impredicative instantation of 'coerce': op = coerce op -- where op has a higher rank type See Note [Newtype-deriving instances] in TcGenDeriv. Cure. 1. It is ghastly to rely on ImpredicativeTypes (a 100% flaky feature) to instantiate coerce polymorphically. Happily we now have Visible Type Application, so I've used that instead which should be solid and reliable. 2. I deleted the code in tc_sub_type_ds that allows the constraint solver to "look through" a unification variable to find a polytype. That used to be essential in the days of ReturnTv, but it's utterly unreliable and should be consigned to the dustbin of history. (We have ExpType now for the essential uses.) Tests involving ImpredicativeTypes are affected, but I'm not worried about them... it's advertised as a feature you can't rely on, and I want to reform it outright.
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