Bogus -Woverlapping-patterns warning with OverloadedStrings

$ cat Test.hs
{-# LANGUAGE OverloadedStrings, LambdaCase #-}
import Data.String

data Expr = App Expr Expr | Var String
  deriving (Eq)

instance IsString Expr where
  fromString = Var . fromString

go = \case
  App ( App ( App "refWithFile" identM ) filenameM) exceptionMayM -> Just 2
  App ( App "and" a ) b -> Just 3
  App ( App "or" a ) b -> Just 4
  _ -> Nothing

go' = \case
  App ( App ( App "refWithFile" identM ) filenameM) exceptionMayM -> Just 2
  App ( App "and" a ) b -> Just 3
  _ -> Nothing

go'' = \case
  App ( App ( App (Var "refWithFile") identM ) filenameM) exceptionMayM -> Just 2
  App ( App (Var "and") a ) b -> Just 3
  App ( App (Var "or") a ) b -> Just 4
  _ -> Nothing

main = do
  let expr = App (App "or" "a") "b"
  print (go expr)
  print (go' expr)

$ runghc-8.4.3 Test.hs

Test.hs:13:3: warning: [-Woverlapping-patterns]
    Pattern match is redundant
    In a case alternative: App (App "or" a) b -> ...
   |
13 |   App ( App "or" a ) b -> Just 4
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Just 4
Nothing
$ runghc-8.6.1 Test.hs

Test.hs:13:3: warning: [-Woverlapping-patterns]
    Pattern match is redundant
    In a case alternative: App (App "or" a) b -> ...
   |
13 |   App ( App "or" a ) b -> Just 4
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Just 4
Nothing

The pattern match checker complains about the "or" case of go being redundant, but, when it is removed (as it is in go') the output is different. go'' demonstrates that OverloadedStrings is relevant, as that is *not* generating a warning. Removing either of the other two cases of go also suppresses the warning: all three are necessary.

As seen in the transcript, this is happening on both 8.4.3 and 8.6.1.

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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