This bug was thrown up by Trac #11361, but I found that the
problem was deeper: GHC was allowing

  class C a where
    type F (a :: k) :: *
    type F (x :: *) = x    -- Not right!

(Which is now indexed-types/should_compile/T11361a.)

Anyway the fix is relatively simple; use tcMatchTys in
tcDefaultAssocDecl.

Merge to 8.0 branch.