#13651: Invalid redundant pattern matches with -XTypeFamilyDependencies -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple InjectiveFamilies | Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code triggers a warning about a redundant pattern match for `foo`: {{{ {-# LANGUAGE TypeFamilies, TypeFamilyDependencies #-} type family F r s = f | f -> r s type instance F (Bar h (Foo r)) (Bar h (Foo s)) = Bar h (Bar r s) data Bar s b data Foo a foo :: (F cr cu ~ Bar h (Bar r u), F cu cs ~ Bar (Foo h) (Bar u s)) => Bar h (Bar r u) -> Bar (Foo h) (Bar u s) -> Foo (cr -> cs) foo = undefined }}} {{{ warning: [-Woverlapping-patterns] Pattern match is redundant In an equation for ‘foo’: foo = .. }}} This warning seems invalid to me: I'm not sure how a single definition could constitute a redundant pattern match. Moreover, if I try to address the warning by removing the "redundant" pattern, the code (of course) fails to compile because there is no accompanying binding for the signature of `foo`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13651 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler