a84d1b21ae
only check cg defaults wf once instantiated the previous fixmes here didn't make too much sense as I didn't yet fully understand the code further below. That code only runs if the predicates using our generic param default are fully concrete after substituting our default, which never happens if our default is generic. r? `@oli-obk` `@BoxyUwU`
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.