Allow class associated types to reference functional dependencies

Consider:

class Foo i j | i -> j where
   type Bar i :: *
   type Bar i = j

This is rejected as j is not an argument to Bar, but when we realize any instance j will be a known type, determined by i, as told to us by the | i -> j functional dependency on the class.

This would likely require some sort of extra pass over the instances with class associated types of this form to elaborate them, or a slight modification of the default signature implementation for class associated types.

Trac metadata

Trac field	Value
Version	7.10.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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