The following program throws a heap overflow if you compile it with optimization on GHC 8.8 and HEAD:

module Bug where

import Data.Bits (setBit)

f :: Int
f = foldl setter 0 $ zip [0..] [()]
  where
    setter v (ix, _) = setBit v ix

$ ~/Software/ghc3/inplace/bin/ghc-stage2 -fforce-recomp -O Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.9.20190309 for x86_64-unknown-linux):
        heap overflow

This does not happen with GHC 8.6.4.

The FontyFruity package on Hackage fails to build on GHC HEAD due to this regression.