[GHC] #15713: Bogus -Woverlapping-patterns warning with OverloadedStrings

6 Oct 2018

      #15713: Bogus -Woverlapping-patterns warning with OverloadedStrings
-------------------------------------+-------------------------------------
           Reporter:                 |             Owner:  (none)
  quasicomputational                 |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 $ 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.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15713
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler