Incorrect non-exhaustive pattern warning with PatternSynonyms

{-# language PatternSynonyms #-}
{-# language ViewPatterns #-}
{-# language TypeFamilies #-}

module Bug where

newtype GenLocated e = L e

newtype WithSourceText a = WithSourceText a

type StringLiteral = WithSourceText String

{-# COMPLETE StringLiteral #-}
pattern StringLiteral sl_s = WithSourceText sl_s

type family SrcSpanLess a

class HasSrcSpan a where
  decomposeSrcSpan :: a -> GenLocated (SrcSpanLess a)

type instance SrcSpanLess (GenLocated e) = e

instance HasSrcSpan (GenLocated a) where
  decomposeSrcSpan = id

bug :: GenLocated StringLiteral -> String
bug (decomposeSrcSpan->L (StringLiteral s)) = s

workaround :: GenLocated StringLiteral -> String
workaround (decomposeSrcSpan->L sl) = s
  where StringLiteral s = sl

$ ghci -Wincomplete-patterns Bug.hs
GHCi, version 8.7.20190103: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/simon/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:27:1: warning: [-Wincomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘bug’: Patterns not matched: _
   |
27 | bug (decomposeSrcSpan->L (StringLiteral s)) = s
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Ok, one module loaded.

I've been able to reproduce the issue with all versions of GHC >= 7.8.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture