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Bogus -Woverlapping-patterns warning with OverloadedStrings

    • Closed Issue created by quasicomputational @trac-quasicomputational 
    $ 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
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
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