GHC suggests -XIncoherentInstances when it's already on

If I say

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE IncoherentInstances #-}

-- All instances are incoherent because of -XIncoherentInstances
class CallResult r where
instance {-# OVERLAPPABLE #-} CallResult (m b) where
instance {-# OVERLAPPABLE #-} CallResult x where

call :: (CallResult r) => r
call = undefined

partial = call

(reduced from an attempt to write a variadic call function)

I get

Variadic.hs:12:11: error:
    • Overlapping instances for CallResult r0
        arising from a use of ‘call’
      Matching instances:
        instance [overlappable] CallResult x -- Defined at Variadic.hs:7:31
        instance [overlappable] CallResult (m b)
          -- Defined at Variadic.hs:6:31
      (The choice depends on the instantiation of ‘r0’
       To pick the first instance above, use IncoherentInstances
       when compiling the other instance declarations)
    • In the expression: call
      In an equation for ‘partial’: partial = call
   |
12 | partial = call
   |    

Notice that I have used -XIncoherentInstances.