haddock error if reexport pattern of data family instance.

Summary

haddock error if reexport pattern of data family instance.

Steps to reproduce

src/A.hs

{-# LANGUAGE PatternSynonyms #-}
module A (pattern BarUnit) where import B

src/B.hs

{-# LANGUAGE PatternSynonyms, TypeFamilies #-}
module B (pattern BarUnit) where

class Foo a where data Bar a
instance Foo () where data Bar () = BarUnit

run

% stack haddock
...
haddock: panic! (the 'impossible' happened)
  (GHC version 8.10.1:
        extractDecl

Ambigous decl for BarUnit in class:
   class Foo a where
      data Bar a
Matches:
   [] []
Call stack:
   CallStack (from HasCallStack):
     callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable
     pprPanic, called at utils/haddock/haddock-api/src/Haddock/Interface/Create.hs:1078:16 in main:Haddock.Interface.Create

Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug

...

Expected behavior

It create document.

Environment

• GHC version used: 8.10.1
• Stackage: nightly-2020-08-03

Optional:

• Operating System: Gentoo GNU/Linux
• System Architecture: