#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): Okey I think I see where the Overlapping instances come from in the case of KnownNat and KnownSymbol. Let me take the following code sample {{{#!haskell signature Abstract where data Foo :: Nat instance KnownNat Foo module Util where import Abstract foo = natVal (Proxy :: Proxy Foo) module Concrete where type Foo = Int module Main where {- mixin Concrete and get the ConcreteUtil -} }}} In the main module there are two dictionaries for `KnownNat Foo`, one which comes via Util (which incidently is actually coming from Concrete and the other directly coming from Concrete. But these two are "different" in the sense that the old code generates KnownNat on the fly and these two generations manifest as two different instances. Is there some way this hypothesis can be tested out say be tracing the compilation for the example ? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15379#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler