#12466: Typechecker regression: Inaccessible code in a type expected by the context -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): As described in [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts... #class-default-signatures the manual], a generic-default type signature (that is, one using the `default` keyword in a class definition) is ''more specific'' than the "normal" type signature for a class method. When no implementation is given for a method in an instance declaration, the default implementation is typechecked against the generic-default type signature. In the code in the Description, this typecheck fails. On the other hand, if we provide an implementation for `foo`, the generic- default signature is not consulted in the instance declaration, and so there is no problem. When you add an `InstanceSigs` signature, then you're just writing a fresh signature that fails to typecheck. Your new signature is utterly unrelated to the generic-default signature. Does this help to clarify? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12466#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler