Uncovered null issue. Workaround turns out to result in the same wonky existential as before. Work is a bit desperate now but there are still a couple of combinations of previous attempts to try.
Trying to shuffle some previous attempts together. Alas.
Gah! Lhs, not rhs! 'T' now looks sort of right, but annotation on 'value' still doesn't seem to stick.
Trying scala.annotation.TypeConstraint, but does not look like it does anything here.