#8714: ExistentialQuantification plus DeriveDataTypeable leads to core lint error -------------------------------------+---------------------------------- Reporter: CoreyOConnor | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 (amd64) Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+---------------------------------- Comment (by goldfire): I've tracked this one down, I know exactly what's going wrong, and I know how to fix this particular case, but I'm worried that this may be representative of a class of errors, and I'm not sure how to fix the lot of them. The problem is that !SetLevels, while doing various core transformations (which I don't yet understand), builds up a substitution. This subst is applied as the `CoreExpr` tree is traversed. But, some bits are crucially left out. The bug in this report is because the binder of a `case` statement is left out -- its type is a type variable, but the type variable is substed during the transformation. The result: a binder whose type contains an out-of-scope type variable. The solution, of course, is to make sure to apply the subst to the binder's type. For example, the following works: {{{ = do { let substed_bndr = setVarType case_bndr (substTy (le_subst env) (varType case_bndr)) case_bndr' = TB substed_bndr bndr_spec }}} These lines should be in place of the current line 385, which sets `case_bndr'` without a subst. But, a casual look around tells me that lambda-bound binders are also seemingly left out of the substitution, and I see that there is a `cloneVar` function which seems to be applying the subst. Perhaps the fix should use `cloneVar`. In any case, there's more going on here than I feel comfortable fiddling with, but a cognoscente should be able to use my detective work to squash the bug in rather little time. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8714#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler