Cannot parse Haddock comment on GADT argument with strictness annotation

Summary

Originally reported in the Haddock issue tracker: https://github.com/haskell/haddock/issues/1053. The issue is that GHC won't parse a Haddock comment on a GADT constructor argument that already has a strictness annotation.

Steps to reproduce

$ cat GADT.hs
data F a where
  X ::
      !Int -- ^ comment
   -> F Int
$ ghc -haddock GADT.hs
[1 of 1] Compiling Main             ( GADT.hs, GADT.o )

GADT.hs:3:7: error:
    • Unexpected strictness annotation: !Int
      strictness annotation cannot appear nested inside a type
    • In the type ‘!Int " comment"’
      In the definition of data constructor ‘X’
      In the data declaration for ‘F’
  |
3 |       !Int -- ^ comment
  |       ^^^^

Expected behavior

The input file GADT.hs should parse just fine!

Environment

GHC versions where this reproduces: 8.9.20190224, 8.6.4.

8.4.4, 8.2.2, and 8.0.2 also won't parse, but with different error messages.