#15110: Exitification abstracts over shadowed variables -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Simon did some code review of `Exitify.hs` in https://ghc.haskell.org/trac/ghc/ticket/14152#comment:36 and he was onto something; the code is wrong after all: Consider this core term {{{ joinrec j (x :: Bool) (x :: Int) = if x == 0 then foo x else jump j True (x - 1) in … }}} where the two parameters `x` and `x` are different variables, but shadow each other (i.e. have the same unique). This is legal Core! But Exitification chooses the variables to abstract over in this line: {{{ -- The possible arguments of this exit join point -- No need for `sortQuantVars`, `captured` is already in dependency order abs_vars = map zap $ filter (`elemVarSet` fvs) captured }}} where `captured = [x::Bool,x::Int]` in this case, which would yield this code {{{ join exit (x :: Bool) (x :: Int) = foo x in joinrec j (x :: Bool) (x :: Int) = if x == 0 then jump exit (x::Bool) (x::Int) else jump j True (x - 1) in … }}} which is no longer type correct (or even “scope-correct”) any more, as it passes the `Int`-typed `x` as the `Bool` argument to `exit`. The conditions where this happens, i.e. such shadowing, is rather obscure so I did not even try to produce a lint error here. It also means that this is rather low severity, but I’ll still go and fix it. An application of `reverseNub` seems to be in order – or simply switching to keeping `captured` in a `DVarSet` rather than a list (in which case `sortQuantVars` would be required…) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15110 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler