Zubin pushed to branch wip/27627 at Glasgow Haskell Compiler / GHC Commits: fd04b1ba by Simon Peyton Jones at 2026-08-25T12:27:11+05:30 Never make an absent filler for a constraint type mkAbsentFiller used isTerminatingType to decide, but that is not enough. Consider class Eq a => UC a where {} let u :: UC Int -- UC Int is a "non-terminating type" u = error "Absent" let e :: Eq Int -- Eq Int is a "terminating type" e = $p1UC u We clearly must not make a filler for `e`, because we speculatively evaluate it. But speculatively evaluating `e` forces `u`, so we must not make one for `u` either. Asking isDictTy instead is not enough either, because it does not catch a constraint hidden behind an unreduced type family application: type family F a :: Constraint type instance F W = TC W a :: F W => Int -> Int -- (F W) argument is absent Oops! Entered absent arg Arg: irred Type: F W So play safe and use isPredTy: never make an absent filler for any constraint-kinded type. Fixes #27627 - - - - - 6646e259 by Zubin Duggal at 2026-08-25T12:27:11+05:30 Add tests for absent fillers at dictionary types T27627 a unary class whose superclass is a non-unary class T27627a ...whose superclass is a Constraint-kinded type family T27627b ...whose superclass is a quantified constraint T27627c a unary class applied to itself, (UC (UC (TC a))) T27627e a (forall b. P b) dictionary that loops - - - - - 5a26c3cd by Zubin Duggal at 2026-08-25T12:27:11+05:30 An abstract TyCon may hide a unary class A class declared in an hs-boot file is an AbstractTyCon inside the module loop, and compiling the real declaration may reveal it to be a UnaryClassTyCon. - isTerminatingType returned True for such AbstractTyCons - IfaceToCore set the unary flag to False in the DFunId So we could end up speculating bottom dictionaries because inside a module loop we see an UnaryClassTyCon as an AbstractTyCon Use mayBeUnaryClassTyCon instead of isUnaryClassTyCon, which returns True for an abstract TyCon. Fixes #27704 - - - - - b1b7d5d2 by Zubin Duggal at 2026-08-25T12:27:11+05:30 Specialise: don't replace dead args with absent fillers specHeader decides an argument is dead by calling isDeadBinder on a binder of the /optimised RHS/, then applies the filler to the /stable unfolding/ template instead. The two may differ, so the argument can be dead in the RHS and not in the template. The specialised function's unfolding then has an absent filler, and any call site that inlines it evaluates the error thunk. Dropping dead args in the specialiser is rarely worth it, to quote Simon, "The later worker/wrapper pass will pick up the dead arg later if it is really dead. Keeps the specialiser simpler." So instead of trying to check if the arg really is dead in the stable unfolding, just drop the logic for dropping dead args in the specialiser altogeher. Fixes #27703 - - - - - 283ef4f1 by Zubin Duggal at 2026-08-25T12:27:11+05:30 CorePrep: don't speculate a call across an hs-boot edge We take care not to evaluate things that might be bottom, like a looping dictionary group, but our analysis is defeated by boot files. We only track recursion within a module, so two dictionaries that depend on each other across a module loop each look non-recursive, and we might speculate them. Any recursion we cannot see must cross an hs-boot edge, so refuse to speculate calls that cross one. Fixes #27717 - - - - - 8b18df0a by Zubin Duggal at 2026-08-25T12:27:11+05:30 fixup! Specialise: don't drop a dead arg that the stable unfolding uses - - - - - 7c8fd45d by Zubin Duggal at 2026-08-25T12:27:11+05:30 fixup! An abstract TyCon may hide a unary class - - - - - 4caa0c19 by Zubin Duggal at 2026-08-25T12:27:11+05:30 fixup! CorePrep: don't speculate a call across an hs-boot edge - - - - - 66 changed files: - + changelog.d/27627 - + changelog.d/27703 - + changelog.d/27704 - + changelog.d/27717 - compiler/GHC/Core.hs - compiler/GHC/Core/Opt/Specialise.hs - compiler/GHC/Core/Opt/WorkWrap/Utils.hs - compiler/GHC/Core/Predicate.hs - compiler/GHC/Core/TyCon.hs - compiler/GHC/Core/Type.hs - compiler/GHC/CoreToStg/Prep.hs - compiler/GHC/IfaceToCore.hs - compiler/GHC/Types/Demand.hs - compiler/GHC/Types/Literal.hs - + testsuite/tests/core-to-stg/T27627/Callee.hs - + testsuite/tests/core-to-stg/T27627/Caller.hs - + testsuite/tests/core-to-stg/T27627/Main.hs - + testsuite/tests/core-to-stg/T27627/T27627.stdout - + testsuite/tests/core-to-stg/T27627/all.T - + testsuite/tests/core-to-stg/T27627a/Callee.hs - + testsuite/tests/core-to-stg/T27627a/Caller.hs - + testsuite/tests/core-to-stg/T27627a/Main.hs - + testsuite/tests/core-to-stg/T27627a/T27627a.stdout - + testsuite/tests/core-to-stg/T27627a/all.T - + testsuite/tests/core-to-stg/T27627b/Callee.hs - + testsuite/tests/core-to-stg/T27627b/Caller.hs - + testsuite/tests/core-to-stg/T27627b/Main.hs - + testsuite/tests/core-to-stg/T27627b/T27627b.stdout - + testsuite/tests/core-to-stg/T27627b/all.T - + testsuite/tests/core-to-stg/T27627c/Callee.hs - + testsuite/tests/core-to-stg/T27627c/Caller.hs - + testsuite/tests/core-to-stg/T27627c/Main.hs - + testsuite/tests/core-to-stg/T27627c/T27627c.stdout - + testsuite/tests/core-to-stg/T27627c/all.T - + testsuite/tests/core-to-stg/T27627e.hs - + testsuite/tests/core-to-stg/T27627e.stdout - + testsuite/tests/core-to-stg/T27627f/Callee.hs - + testsuite/tests/core-to-stg/T27627f/Caller.hs - + testsuite/tests/core-to-stg/T27627f/Inst.hs - + testsuite/tests/core-to-stg/T27627f/Main.hs - + testsuite/tests/core-to-stg/T27627f/T27627f.stdout - + testsuite/tests/core-to-stg/T27627f/all.T - + testsuite/tests/core-to-stg/T27704/Callee.hs - + testsuite/tests/core-to-stg/T27704/Callee.hs-boot - + testsuite/tests/core-to-stg/T27704/Main.hs - + testsuite/tests/core-to-stg/T27704/Mid.hs - + testsuite/tests/core-to-stg/T27704/T27704.stdout - + testsuite/tests/core-to-stg/T27704/all.T - + testsuite/tests/core-to-stg/T27704a/Callee.hs - + testsuite/tests/core-to-stg/T27704a/Callee.hs-boot - + testsuite/tests/core-to-stg/T27704a/Main.hs - + testsuite/tests/core-to-stg/T27704a/Mid.hs - + testsuite/tests/core-to-stg/T27704a/T27704a.stdout - + testsuite/tests/core-to-stg/T27704a/all.T - + testsuite/tests/core-to-stg/T27717/Callee.hs - + testsuite/tests/core-to-stg/T27717/Callee.hs-boot - + testsuite/tests/core-to-stg/T27717/Main.hs - + testsuite/tests/core-to-stg/T27717/Mid.hs - + testsuite/tests/core-to-stg/T27717/T27717.stdout - + testsuite/tests/core-to-stg/T27717/Ty.hs - + testsuite/tests/core-to-stg/T27717/all.T - testsuite/tests/core-to-stg/all.T - + testsuite/tests/simplCore/should_run/T27703/Lib.hs - + testsuite/tests/simplCore/should_run/T27703/Main.hs - + testsuite/tests/simplCore/should_run/T27703/T27703.stdout - + testsuite/tests/simplCore/should_run/T27703/all.T The diff was not included because it is too large. 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