#15379: Don't reject user-written instances of KnownNat and friends in hsig files -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ppk): Just reporting the things that I learned here (thanks to discussions with @mpickering on IRC) 1. With the minor change, the overlapping instance error is thrown only for inbuilt type classes like `KnownNat` and `KnownSymbol` but ''not for'' Show 2. Overlapping instance is ''not a problem'' for `Typeable`, the other builtin type class that is affected by this change. This means that there is something in the witnessing of `KnownNat` and `KnownSymbol` that is making it see multiple instances. I have updated the test case with all these special classes so that we keep track of these differences. In addition @ezyang just mentioned that it is not clear whether the uniform code will work with both hsboot and hsig files. Since we are not confident about the hsboot part, I will only handle the hsig files in the patches to come. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15379#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler