#15172: Undecidable instances slip through -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider {{{ {-# LANGUAGE FlexibleInstances, TypeFamilies, ConstraintKinds #-} type family F a :: Constraint class C a where instance (F a) => C [[a]] where }}} This should be rejected because the `F a` constraint could expand to be arbitrarily large, depending on `F`. There's simply a missing check in `checkInstTermination`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15172 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler