#14462: deriving on associated data types fails to find constraints -------------------------------------+------------------------------------- Reporter: mf825 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: TypeFamilies, | Operating System: Unknown/Multiple associated types, deriving | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE TypeFamilies, UndecidableInstances #-} class D a where data DT a class C a where data CT a instance (D a, Eq (DT a)) => C (Maybe a) where data CT (Maybe a) = CTMaybe (DT a) deriving (Eq) {- $ stack --resolver=nightly-2017-10-20 exec -- ghc --version The Glorious Glasgow Haskell Compilation System, version 8.2.1 $ stack --resolver=nightly-2017-10-20 exec -- ghci Main.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help (0.00 secs, 0 bytes) (0.00 secs, 0 bytes) (0.00 secs, 0 bytes) (0.00 secs, 0 bytes) (0.00 secs, 0 bytes) (0.00 secs, 0 bytes) (0.00 secs, 0 bytes) (0.00 secs, 0 bytes) Loaded GHCi configuration from /home/mf/.ghci [1 of 1] Compiling Main ( Main.hs, interpreted ) Main.hs:7:48: error: • No instance for (Eq (DT a)) arising from the first field of ‘CTMaybe’ (type ‘DT a’) Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (Eq (CT (Maybe a))) | 7 | data CT (Maybe a) = CTMaybe (DT a) deriving (Eq) | ^^ Failed, 0 modules loaded. (0.03 secs,) Prelude> -- if i remove the offending @deriving@ clause above and add this line, everything is fine. -- use -XFlexibleInstances -XStandaloneDeriving for this. deriving instance Eq (DT a) => Eq (CT (Maybe a)) -} }}} checked on linux with ghc8.0.2 and 8.2.1. thanks! and sorry if i've missed a previous report covering this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14462 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler