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This patch changes how we perform representation-polymorphism checking for primops (and other wired-in Ids such as coerce). When instantiating the primop, we check whether each type variable is required to instantiated to a concrete type, and if so we create a new concrete metavariable (a ConcreteTv) instead of a simple MetaTv. (A little subtlety is the need to apply the substitution obtained from instantiating to the ConcreteTvOrigins, see Note [substConcreteTvOrigin] in GHC.Tc.Utils.TcMType.) This allows us to prevent representation-polymorphism in non-argument position, as that is required for some of these primops. We can also remove the logic in tcRemainingValArgs, except for the part concerning representation-polymorphic unlifted newtypes. The function has been renamed rejectRepPolyNewtypes; all it does now is reject unsaturated occurrences of representation-polymorphic newtype constructors when the representation of its argument isn't a concrete Runtim...
