#8128: Standalone deriving fails for GADTs due to inaccessible code ----------------------------------------------+---------------------------- Reporter: adamgundry | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by simonpj): As you say, the typechecker takes account of context (ie other in-scope equalities) so it's not a simple, local check. Code generated by `deriving` is typechecked separately (line 909 of `TcRnDriver`). So it'd be reasonably easy to suppress "inaccessible code" warnings for generated code. There is a separate check for overlapping or fully-overlapped patterns at the desugaring stage. This currently does NOT take account of GADTs etc, and there are many open tickets as a result. Re-doing the overlapping- pattern check would be an excellent thing. However, by the time code gets to the the overlap check, we've lost track of which code was generated by `deriving`. It'd be possible, although a little tiresome, to retain that information. If someone wants to work on this, I could advise. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8128#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler