#14751: GHC wrongly warns of incomplete pattern matches when using pattern synonyms. -------------------------------------+------------------------------------- Reporter: vanessamchale | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.4.1-alpha1 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: GHC rejects (amd64) | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If one writes the following code: {{{ {-# LANGUAGE PatternSynonyms #-} module Version where newtype Version = Version [Integer] deriving (Eq) pattern V :: [Integer] -> Version pattern V is = Version is instance Ord Version where (V []) <= (V []) = True (V []) <= _ = True _ <= (Version []) = False (V (x:xs)) <= (V (y:ys)) | x == y = Version xs <= Version ys | otherwise = x <= y }}} and compiles with {{{ghc -Wall -Werror}}}, one gets the following message: {{{ [1 of 1] Compiling Version ( Bug.hs, Bug.o ) Bug.hs:12:5: error: [-Wincomplete-patterns, -Werror=incomplete-patterns] Pattern match(es) are non-exhaustive In an equation for ‘<=’: Patterns not matched: _ _ _ _ _ _ _ _ | 12 | (V []) <= (V []) = True | ^^^^^^^^^^^^^^^^^^^^^^^... }}} However, the following code compiles correctly without warnings: {{{ {-# LANGUAGE PatternSynonyms #-} module Version where newtype Version = Version [Integer] deriving (Eq) instance Ord Version where (Version []) <= (Version []) = True (Version []) <= _ = True _ <= (Version []) = False (Version (x:xs)) <= (Version (y:ys)) | x == y = Version xs <= Version ys | otherwise = x <= y }}} This bug appears in GHC 8.2.2 as well as GHC-8.4.1-alpha2. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14751 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler