Polymorphic functions are a little difficult. Suppose the following ApplySig happens: Trait.sig(t). Then we must look up the enclosing function and

1. Check there is an instance of Trait defined
2. If t or the result of applying Trait.sig to t contains an associated type of Trait then we must check if there is an equality constraint of the form Elm[t] ~ t2 and return that (can t2 be polymorphic?)
3. Lastly, if that is not the case we should probably just check that Elm[t] for all instances of Trait is "upcastable" to the desired type.

The Instances phase contains safe ways to deal with trait / instance lookup.

See discussion in #12124