-XImpredicativeTypes implies -XRankNTypes, and improve error msg in TcMType
If you are going for impredicative types you almost certainly want RankN too. The change to TcMType improves the error when you say T (forall a. blah) where T is a type synonym. This doesn't necessarily need impredicativity, if you have LiberalTypeSynonyms.
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