COMPLETE pragma insufficiently documented

Summary

Consider the following program:

data LexemeVW' var white
  = LVWVar' var Var
  | LVWWhite' white Whitespace
  | LLex Lexeme
  deriving (Eq, Show)

type LexemeV = LexemeVW' () Void
type LexemeW = LexemeVW' Void ()
type LexemeVW = LexemeVW' () ()

pattern LVar :: Var -> LexemeVW' () white
pattern LVar v <- LVWVar' () v
  where
    LVar v = LVWVar' () v

pattern LWhite :: Whitespace -> LexemeVW' var ()
pattern LWhite v <- LVWWhite' () v
  where
    LWhite v = LVWWhite' () v

isV :: LexemeV -> Bool
isV (LVar _) = True
isV (LLex _) = False

The matches on isV are complete (since the LVWWhite' constructor is prohibited by having a Void argument). I'd like to add a COMPLETE pragma informing GHC of this, but, this is only complete for LexemeV, not LexemeW or LexemeVW! Presumably this pragma will do the trick:

{-# COMPLETE LVar, LLex :: LexemeVW' () Void #-}

But this is a parse error!

    parse error on input ‘(’
    |
486 | {-# COMPLETE LVar, LLex :: LexemeVW' () Void #-}
    |                                      ^

Steps to reproduce

Small repro:

{-# LANGUAGE PatternSynonyms #-}
import Data.Void

type Var = Int
type Whitespace = Int
type Lexeme = Int

data LexemeVW' var white
  = LVWVar' var Var
  | LVWWhite' white Whitespace
  | LLex Lexeme
  deriving (Eq, Show)

pattern LVar :: Var -> LexemeVW' () white
pattern LVar v <- LVWVar' () v
  where
    LVar v = LVWVar' () v
{-# COMPLETE LVar, LLex :: LexemeVW' () Void #-}

Expected behavior

I expect the COMPLETE pragma to parse, and to subsequently silence the isV warning.

Environment

• GHC version used: 8.8.1 and 8.6.5

Optional:

• Operating System:
• System Architecture:

EDIT (by @rae): The problem is in the "I expect", above, because the manual does not lay this out as clearly as it should.