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 al...
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