Haddock drops {- -} comments

Summary

Haddock drops {- -} comments inside a haddock comment itself delimited with {- | ... -}.

My understanding is that the GHC API is responsible for parsing this, but please let me know if this is wrong and I should file a haddock issue instead.

Steps to reproduce

Process the following module with haddock:

{- |
A Haskell comment looks like this: {- comment -}
-}
module Test where

I expect the full doc string to show up in html, but the {- comment -} part is dropped.

Environment

• GHC version used:

Tested with 8.6 and 8.8.

As a side note, I tried to reproduce this bug using raw ghc api like this:

import GHC
import GHC.Paths
import Outputable
import GhcMonad

main =
  runGhc (Just libdir) $ do
    dynflags <- getSessionDynFlags
    setSessionDynFlags dynflags { packageFlags = [] }
    setTargets [Target (TargetFile "Test.hs" Nothing) False Nothing]
    load LoadAllTargets
    ms <- getModSummary (mkModuleName "Test")
    pm <- parseModule ms
    let pp = showPpr dynflags (hsmodHaddockModHeader . unLoc $ pm_parsed_source pm)
    liftIO $ putStrLn pp

But I got Nothing — i.e. the parsed module does not have hsmodHaddockModHeader. I'd be interested in learning what I did wrong there.