5eca626e40
We used to avoid doing this because we didn't want to make coercion depend on the state of inference. For better or worse, we have moved away from this position over time. Therefore, I am going to go ahead and resolve the `b` target type early on so that it is done uniformly. (The older technique for managing this was always something of a hack regardless; if we really wanted to avoid integrating coercion and inference we needed to be more disciplined about it.)
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.