tcfail138 compiles but shouldn't

[daniel.is.fischer]

Or should it now? Happened before:

-- Note: Sept 08: when fixing Trac #1470, tc138 started working!
-- This test is a very strange one (fundeps, undecidable instances),
-- so I'm just marking it as "should-succeed".  It's not very clear to
-- me what the "right" answer should be; when we have the type equality
-- story more worked out we might want to think about that.

[simonpj]

Done