codifies the state of the art wrt SI-8104

As it was discovered in SI-8104, whiteboxity doesn’t apply equally to
type parameters and type members of materialized type classes.

During implicit search and subsequent type inference, whitebox type parameters
are consistently erased to wildcards, whereas whitebox type members sometimes
remain as is and get in the way of signature conformance checks.
Merge pull request #3311 from xeno-by/topic/fine-points-of-whiteboxity-master

(master) codifies the state of the art wrt SI-8104