Disabling warnings breaks instance resolution

This snippet:

{-# OPTIONS_GHC -Wno-all #-}
main = print $ Just () == Nothing

fails with

<source>:2:24: error: [GHC-39999]
    * No instance for `Eq (Maybe ())' arising from a use of `=='
    * In the second argument of `($)', namely `Just () == Nothing'
      In the expression: print $ Just () == Nothing
      In an equation for `main': main = print $ Just () == Nothing
  |
2 | main = print $ Just () == Nothing
  |       

Removing -Wno-all makes compilation succeed.

I checked multiple GHC versions and the last one that didn't have this rather peculiar behavior is GHC-8.4.4.

I get a different error message locally:

    • Overlapping instances for Eq (Maybe Int)
        arising from a use of ‘==’
      Matching instance:
        instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Maybe’
      There exists a (perhaps superclass) match:
      (To pick the first instance above, use IncoherentInstances
       when compiling the other instance declarations)

which also goes away if I remove -Wno-all. This is with GHC-9.6.6.