#8503: New GeneralizedNewtypeDeriving check still isn't permissive enough -------------------------------------+------------------------------------ Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: #8541 -------------------------------------+------------------------------------ Comment (by nomeata): Have you checked that it fails? I believe it would work: {{{ Coercible (Maybe (Fix Maybe)) (Fix Maybe) }}} is only matched by {{{ instance Coercible a (Maybe (Fix Maybe) => Coercible a (Fix Maybe) }}} so we are left with {{{ Coercible (Maybe (Fix Maybe)) (Maybe (Fix Maybe)) }}} which is solved by the reflexive instance {{{ Coercible a a }}} (I’m not claiming that all cases work as smoothly, but they need to be a bit more constructed :-)) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8503#comment:22 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler