#16346: Lint error on master: Occurrence is GlobalId, but binding is LocalId -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 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 simonpj): Here's why the second crash happens. We start with {{{ case M.f_2dr[GblId] of f' { W# y -> let f_2dr[LclId] = f' in case M.f_2dr[GblId] of { W# z -> blah }}} (All this is in the body of `$s$fGBinaryGetTYPE:+:1_sfOT` in `Distribution.Types.ComponentRequestedSpec`.) Then * It turns out that `M.f_2dr` is a HNF so the first case can vanish. We extend the substitution with `f' :-> M.f_2dr[GblId]`. * We simplify the RHS of the `let` to get `M.f_2dr[GblId]` * We simplify the let-binder. It's not in scope, so we don't mess with its unique, and we extend the in-scope set with `M.f_2dr[LclId]`. * Now post-inline-unconditionally fires, so we extend the substitution with `f_2dr :-> M.f_2dr[GblId]` * Now at the occurrence of `M.f_2dr[GblId]` we find it in the substitution, mapping to, well, `M.f_2dr[GblId]`. * Then we look that up in the in-scope set, and get `M.f_2dr[LclId]` * And that is not in scope. What a mess. My conclusion: it's a total disaster to have a local binding for a `LocalId` that shares a unique with a `GlobalId`. We have multiple hacks to avoid trouble, and they don't even work. Let's stop doing it. See #16296. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16346#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler