Apparent infinite loop producing error message with UndecidableSuperClasses

The code below causes the compiler to start printing out an error and then (apparently) loop indefinitely or run out of memory.

{-# LANGUAGE FlexibleContexts, FlexibleInstances, AllowAmbiguousTypes,
                         UndecidableSuperClasses #-}
module M where

data I = I

class All x => All x

class AllZip2 f
instance AllZip2 f

f1 :: (All x, AllZip2 I) => x -> ()
f1 = f2

f2 :: AllZip2 f => x -> ()
f2 _ = ()

I've looked at #11523, and while this seems similar I don't think it's quite the same as I don't see an infinite tower of superclasses, and I don't think it does ever give up and complain about too many iterations.

The error itself is probably genuine, as at the very least the type of f in the call to f2 is ambiguous, but it'd be nice to get it :-)

The output I get before the hang is something like this, I also checked GHC 8.6.4 and GHC 8.7.20190115:

GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling M             ( M.hs, interpreted )

M.hs:12:6: error:

The example is cut down from some real code that uses generics-sop, first by making a minimal example that still uses generics-sop and then by inlining and cutting down the library itself. I've also reported it on their tracker.

The seemingly nonsensical class All x => All x originates from this code in generics-sop where SListI xs = All Top xs is a superclass of All c xs.