Valid hole fits and -XExtendedDefaultRules don't get along

If I say

{-# LANGUAGE ExtendedDefaultRules #-}

module Bug where

blah :: Show a => a -> String
blah x = show _

I get

Bug.hs:6:15: error:
    • Found hole: _ :: ()
    • In the first argument of ‘show’, namely ‘_’
      In the expression: show _
      In an equation for ‘blah’: blah x = show _
    • Relevant bindings include
        x :: a (bound at Bug.hs:6:6)
        blah :: a -> String (bound at Bug.hs:6:1)
      Constraints include Show a (from Bug.hs:5:1-29)
      Valid hole fits include
        () :: () (bound at <wired into compiler>)
        mempty :: forall a. Monoid a => a
          with mempty @()
          (imported from ‘Prelude’ at Bug.hs:3:8-10
           (and originally defined in ‘GHC.Base’))
        maxBound :: forall a. Bounded a => a
          with maxBound @()
          (imported from ‘Prelude’ at Bug.hs:3:8-10
           (and originally defined in ‘GHC.Enum’))
        minBound :: forall a. Bounded a => a
          with minBound @()
          (imported from ‘Prelude’ at Bug.hs:3:8-10
           (and originally defined in ‘GHC.Enum’))
  |
6 | blah x = show _
  |           

Note that GHC ends up unifying the type of _ with (), and then suggests variables that can have type ().