#13959: substTyVar's definition is highly dubious -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: 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 RyanGlScott): Well looking at the definition of `mkVarSet`, it's no surprise that things are going awry: {{{#!hs mkVarSet :: [Var] -> VarSet mkVarSet = mkUniqSet mkUniqSet :: Uniquable a => [a] -> UniqSet a mkUniqSet = foldl' addOneToUniqSet emptyUniqSet }}} It's just grabbing the `Unique` of each type variable, and nothing more. Experimentally, when I tried a debugging build of GHC with the modified version of `substTyVar`, it fell over trying to compile `Data.Type.Equality`. I don't have the full assertion failure at the moment, but I recall that there were indeed more `tenvFVs` than what were contained in the `in_scope` set. [http://git.haskell.org/ghc.git/blob/1ee49cb11c7ad1af20c117a5395df96ded9a729f... Here's] a sample call to `mkVarSet` that's propagated through to `substTyVars`. This is used in the logic for `GeneralizedNewtypeDeriving`, where we need to take all the type variables mentioned in the instance head in preparation for the substitution that substitutes the class tyvars with the types used when deriving the instance. ...Although now that I think about it, this probably isn't a very good example of the thing we're worried about in this ticket, since that substitution is formed by simply zipping the class tyvars with their instantiated types in the instance, so there's no need to ever substitute in the kind of a tyvar... I think. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13959#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler