constraint deduction bug?

[Gabor Greif]

In context of a bigger source file I got this error from GHC (HEAD)

    Could not deduce (Intersect
                        [KeySegment] (BuriedUnder sub k ('Empty
[KeySegment])) inv
                      ~ 'Empty [KeySegment])
    from the context (Intersect
                        [KeySegment] (BuriedUnder sub k ('Empty
[KeySegment])) inv
                      ~ 'Empty [KeySegment])
      bound by a pattern with constructor
                 Nil :: forall (k :: BOX). Sing (Inventory k) ('Empty k),
               in a pattern binding in
                    'do' block

Can somebody confirm that this constraint should be trivially deducible from
the context?

Then I'll try to come up with a reduced repro case.

Cheers,

	Gabor