#11361: Test tc253 doesn't pass with reversed uniques -------------------------------------+------------------------------------- Reporter: niteria | Owner: niteria Type: bug | Status: new Priority: normal | Milestone: 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 niteria): I believe the problem is here: https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/typeche... There's no reason to believe that `ktvs` and `mkTyVarTys fam_tc_tvs` will be in the matching order, and indeed `ktvs` is in the order of uniques. We start out with: {{{ type Fam a_a18o3H b_a18o3G :: * type Fam a_a18o3F x_a18o3E = FamHelper a_a18o3F x_a18o3E }}} then when we reach `tcDefaultAssocDecl` we have: `FamHelper (a_a18o3F |> <*>_N) (x_a18o3E |> <*>_N) |> <*>_N)` `ktvs` is `[x_a18o3E, a_a18o3F]` `mkTyVarTys fam_tc_tvs` is `[a_a18o3H, b_a18o3G]` When we zip them together we end up with a definition that flips the arguments. From there it goes downhill, because when we try to compute `Fam Int Bool` we do it with flipped arguments: `FamHelper Bool Int` which is `(String, Bool)` which eventually doesn't unify with `Maybe Int`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11361#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler