#10009: type inference regression when faking injective type families -------------------------------------+------------------------------------- Reporter: aavogt | Owner: Type: bug | Status: new Priority: high | Milestone: 7.10.2 Component: Compiler (Type | Version: 7.10.1-rc1 checker) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | Blocking: Blocked By: | Differential Revisions: Related Tickets: #10226 | -------------------------------------+------------------------------------- Comment (by jeltsch): I have an issue with GHC 7.10.1, which is probably rooted in the same bug. The following code is accepted by GHC 7.8.3, but not 7.10.1: {{{ {-# LANGUAGE TypeFamilies #-} type family F a :: * type family G b :: * x :: G (F a) ~ a => F a x = undefined }}} GHC 7.10.1 gives the following error message: {{{ Could not deduce (F a0 ~ F a) from the context (G (F a) ~ a) bound by the type signature for x :: (G (F a) ~ a) => F a at Test.hs:7:6-23 NB: ‘F’ is a type function, and may not be injective The type variable ‘a0’ is ambiguous In the ambiguity check for the type signature for ‘x’: x :: forall a. (G (F a) ~ a) => F a To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature for ‘x’: x :: G (F a) ~ a => F a }}} Is this code accepted by GHC HEAD? I came across this problem when trying to recompile the `incremental- computing` package with GHC 7.10.1. Since this issue breaks `incremental- computing` in a nontrivial way, I would be ''extremely'' happy if the fix of this bug would make it into GHC 7.10.2. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10009#comment:29 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler