Documentation: alleged UndecidableInstances loopiness doesn't loop

The example code in the User Guide for the UndecidableInstances extension that is alleged "would make the typechecker loop", doesn't:

    class D a
    class F a b | a->b
    instance F [a] [[a]]
    instance (D c, F a c) => D [a]   -- 'c' is not mentioned in the head

That is with

fD :: (D x, Show x) => x -> String
fD x = show x

y = fD [[()]]

The constraint checking from wanted D [[()]] to F [()] c strips away one [ ]. The improvement F [()] [[()]] puts it back. Presumably the D c constraint i.e. D [[()]] says: oh, I've already seen a wanted like that, no need to check it again. (Perhaps it didn't used to be so smart.)

Change that D instance to this, and the typechecker does loop

instance (D c, F [a] c) => D [a]  

GHC 8.6.4 on Windows architecture.

(The other example discussed there, with the "somewhat strange" definition using .*. does indeed cause loopiness.)

P.S. might it be an idea to offer a documentation template? The 'bug' template didn't seem to apply. I've put a 'documentation' Label.