Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: eb1dcd4d by sheaf at 2026-08-07T17:50:40-04:00 mkWpFun_FRR: fix ordering of coercion composition When the subsumption machinery generates an eta-expansion, we must perform a representation polymorphism check to ensure the lambda binder it introduces has a fixed runtime representation. This is done in GHC.Tc.Utils.mkWpFun_FRR. This check involves composing quite a few coercions, arising from representation-polymorphism checks on both the actual and expected argument types. These coercions are then chained using HsWrapper composition, <.>. The ordering of composition was incorrect, leading to the Core Lint failure reported in #27639. This commit fixes that. Fixes #27639 - - - - - 4 changed files: - + changelog.d/T27639 - compiler/GHC/Tc/Utils/Unify.hs - + testsuite/tests/rep-poly/T27639.hs - testsuite/tests/rep-poly/all.T Changes: ===================================== changelog.d/T27639 ===================================== @@ -0,0 +1,10 @@ +section: compiler +issues: #27639 +mrs: !16474 +synopsis: + Stop representation-polymorphism checks from producing a coercion that fails + Core Lint. +description: + The representation-polymorphism check that is done when subsumption performs + eta-expansion now composes the coercions it builds in the correct order, + avoiding a Core Lint failure. ===================================== compiler/GHC/Tc/Utils/Unify.hs ===================================== @@ -2359,7 +2359,7 @@ mkWpFun_FRR (mb_tc_fun, pos) sub_mult act_af act_mult act_arg act_res exp_af exp act_arg_co (mkNomReflCo act_res) arg_wrap_frr = -- exp_arg_frr ~~> act_arg_frr - mkWpCastN (mkSymCo exp_arg_co) <.> arg_wrap <.> mkWpCastN act_arg_co + mkWpCastN act_arg_co <.> arg_wrap <.> mkWpCastN (mkSymCo exp_arg_co) ; traceTc "mkWpFun_FRR: WpFun" $ vcat [ text "act_mult:" <+> ppr act_mult ===================================== testsuite/tests/rep-poly/T27639.hs ===================================== @@ -0,0 +1,15 @@ +{-# LANGUAGE DeepSubsumption, RankNTypes, PolyKinds, DataKinds, GADTs, TypeOperators, ScopedTypeVariables #-} +module T27639 where + +import GHC.Exts +import Data.Type.Equality + +h :: forall r (a :: TYPE r) (b :: TYPE r). + (r :~: LiftedRep) -> (a :~: b) -> () +h Refl Refl = + let + v :: a -> (Int -> Int) -> Int + v = undefined + u :: b -> (forall c. c -> c) -> Int + u = v + in () ===================================== testsuite/tests/rep-poly/all.T ===================================== @@ -43,6 +43,7 @@ test('T23883c', normal, compile_fail, ['']) test('T23903', normal, compile_fail, ['']) test('T26107', js_broken(22364), compile, ['-O']) test('T26528', normal, compile, ['']) +test('T27639', normal, compile, ['']) test('EtaExpandDataCon', normal, compile, ['-O']) test('EtaExpandStupid1', normal, compile, ['-Wno-deprecated-flags']) View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/eb1dcd4d98548b7bc64c323dc352ddbf... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/eb1dcd4d98548b7bc64c323dc352ddbf... You're receiving this email because of your account on gitlab.haskell.org. Manage all notifications: https://gitlab.haskell.org/-/profile/notifications | Help: https://gitlab.haskell.org/help