#15014: Exhaustivity check should suggest when COMPLETE could be helpful -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.3 Component: Compiler | Version: 8.5 (Type checker) | Keywords: | Operating System: Unknown/Multiple PatternSynonyms, | PatternMatchWarnings | Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{ MacBook-Pro-97:ghc ezyang$ cat A.hs {-# LANGUAGE PatternSynonyms #-} module A where pattern F :: a -> b -> (a, b) pattern F x y = (x, y) g :: (a, b) -> (a, b) g (F x y) = (x, y) MacBook-Pro-97:ghc ezyang$ inplace/bin/ghc-stage2 -c A.hs -Wall -fforce- recomp A.hs:8:1: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In an equation for ‘g’: Patterns not matched: _ | 8 | g (F x y) = (x, y) | ^^^^^^^^^^^^^^^^^^ MacBook-Pro-97:ghc ezyang$ inplace/bin/ghc-stage2 --version The Glorious Glasgow Haskell Compilation System, version 8.5.20180304 }}} Any time the exhaustiveness checker throws up its hands and says that `_` is not matched, we ought to give a hint that this may have occurred due to pattern synonyms, and that the author of the pattern synonyms can help out by specifying a COMPLETE pragma. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15014 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler