Is your bug specific to Fourmolu?

• Yes, I've verified that Ormolu does not have this bug

Did you try it on the web app?

• Yes, I've verified that https://fourmolu.github.io can reproduce this bug

Describe the bug
Provide steps to reproduce the bug, and what the expected behavior should be.

Trying to format this input with import-export-style: leading fails:

module A ( -- | b
c ) where

The error says:

A.hs:1:1
  The GHC parser (in Haddock mode) failed:
  {ErrorWithoutFlag
   parse error on input `c'}

Adding another identifier to the export list fixes the problem. So does removing the Haddock comment. A normal comment isn't sufficient to trigger the bug.