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 |