#6088: GeneralizedNewtypeDeriving + TypeFamilies + Equality constraints -------------------------------------+------------------------------------- Reporter: Lemming | Owner: Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: GHC rejects | Test Case: indexed- valid program | types/should_compile/T6088 Blocked By: | Blocking: Related Tickets: 3046 | -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"03e8d26fe0a8f7981a76550118f3c584cad76c47/ghc" 03e8d26f/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="03e8d26fe0a8f7981a76550118f3c584cad76c47" Prevent GND from inferring an instance context for method-less classes When `GeneralizedNewtypeDeriving` is used with a type class that has no methods, it will generate a redundant context, and as a result, it can trigger warnings when compiled with `-Wredundant-constraints`. This is a simple change in behavior to check beforehand if a class has methods when deriving it with GND, and if it has no methods, avoid inferring the redundant context. Beware that the test for #6088, which used to be expected to fail, now compiles without issue since it doesn't infer a problematic instance context. Thanks to Simon Peyton Jones for doing the necessary refactoring in f05d685ae05ec293083f2fa7ec7ba057fbe64869. Fixes #12814. Test Plan: ./validate Reviewers: goldfire, rwbarton, simonpj, austin, bgamari Reviewed By: simonpj Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2692 GHC Trac Issues: #12814 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6088#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler