EmptyCase thinks pattern match involving type family is not exhaustive, when it actually is
GHC warns on this program:
{-# LANGUAGE EmptyCase #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# OPTIONS_GHC -Wall #-}
module Bug where
import Data.Kind
import Data.Void
data SBool (z :: Bool) where
SFalse :: SBool 'False
STrue :: SBool 'True
type family F (b :: Bool) (a :: Type) :: Type where
F 'True a = a
F 'False _ = Void
dispatch :: forall (b :: Bool) (a :: Type). SBool b -> F b a -> a
dispatch STrue x = x
dispatch SFalse x = case x of {}
$ /opt/ghc/8.2.2/bin/ghci Bug.hs
GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Bug.hs:22:21: warning: [-Wincomplete-patterns]
Pattern match(es) are non-exhaustive
In a case alternative: Patterns not matched: _ :: F b a
|
22 | dispatch SFalse x = case x of {}
| ^^^^
This warning is incorrect, as x
is of type F 'False a
, or Void
, in that case alternative.
Curiously, if you ascribe either the pattern for x
:
dispatch SFalse (x :: F 'False a) = case x of {}
Or the case scrutinee:
dispatch SFalse x = case x :: F 'False a of {}
Then the warning goes away.
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |