During a GHC upgrade we noticed one particular test case seemed to just hang forever (at runtime, not compile time). In searching for a looping bottom value lurking somewhere I found a startling fact, while simplifying an instance declaration:

This exhibited the hanging:

-- Note now-redundant constraint
instance ( Eq (Bar b a)) => Eq (Foo b a) where
  _ == _ = False

This did not (the operation finished promptly):

-- constraint removed
instance Eq (Foo b a) where
  _ == _ = False

We use UndecidableInstances heavily and so this question caught my eye, but I don’t see how the variations above could trigger/resolve an infinite loop like the one discussed there.

My question is how the observed behavior could possibly occur, and an answer would hopefully offer a simple reproducer that builds on the snippet here.

FWIW the change in behavior happens between 9.6.6 and 9.8.2