When I say

{-# LANGUAGE RebindableSyntax #-}
{-# OPTIONS_GHC -fdefer-out-of-scope-variables #-}

module Bug where

foo = 5

I get

Bug.hs:6:7: error: Not in scope: ‘fromInteger’

But I told GHC to defer such errors!

Nobody is really asking for this (I was just trying to abuse GHC for fun), but I think it should be very very easy to implement.