Emphasize mismatching part of complex type mismatches #4578
Description
When a type mismatch is complex, and some types are unbound (is this the correct term?), it can be hard to see where the error actually is. Here is a concrete example I actually wasted quite a bit of time on:
Expected type:
drift.Step(List(String), Input, Output, Nil)
Found type:
drift.Step(List(string), Input, a, b)
It could be made clearer by emphasizing where the type mismatch was exactly:
Expected type:
drift.Step(List(String), Input, Output, Nil)
^^^^^^
Found type:
drift.Step(List(string), Input, a, b)
^^^^^^