Zubin pushed to branch wip/backports-9.12.4 at Glasgow Haskell Compiler / GHC Commits: 451c6a43 by Simon Peyton Jones at 2026-03-04T12:15:09+05:30 Fix a horrible shadowing bug in implicit parameters Fixes #26451. The change is in GHC.Tc.Solver.Monad.updInertDicts where we now do /not/ delete /Wanted/ implicit-parameeter constraints. This bug has been in GHC since 9.8! But it's quite hard to provoke; I contructed a tests in T26451, but it was hard to do so. (cherry picked from commit c052c724d2dfc994994b6548545836969aee8ed8) - - - - - 85b0aae2 by Simon Peyton Jones at 2026-03-04T12:15:09+05:30 Fix subtle bug in GHC.Core.Utils.mkTick This patch fixes a decade-old bug in `mkTick`, which could generate type-incorrect code! See the diagnosis in #26772. The new code is simpler and easier to understand. (As #26772 says, I think it could be improved further.) (cherry picked from commit cbe4300ef586c8bee1800426624db12e0237c6b5) - - - - - 55885e4b by Simon Peyton Jones at 2026-03-04T12:15:09+05:30 Fix long-standing interaction between ticks and casts The code for Note [Eliminate Identity Cases] was simply wrong when ticks and casts interacted. This patch fixes the interaction. It was shown up when validating #26772, although it's not the exactly the bug that's reported by #26772. Nor is it easy to reproduce, hence no regression test. (cherry picked from commit b579dfdc614e288b0fd754ac69ae7ff723d808be) - - - - - 4fdab7ca by sheaf at 2026-03-04T12:15:09+05:30 NamedDefaults: require the class to be standard We now only default type variables if they only appear in constraints of the form `C v`, where `C` is either a standard class or a class with an in-scope default declaration. This rectifies an oversight in the original implementation of the NamedDefault extensions that was remarked in #25775; that implementation allowed type variables to appear in unary constraints which had arbitrary classes at the head. See the rewritten Note [How type-class constraints are defaulted] for details of the implementation. Fixes #25775 Fixes #25778 (cherry picked from commit f1acdd2c2b664ad0bdcaae4064b50e84aa7bc599) - - - - - cfbec95d by Rodrigo Mesquita at 2026-03-04T12:15:09+05:30 bytecode: Use 32bits for breakpoint index Fixes #26325 (cherry picked from commit e368e24779f8a7bf110a025383db23521b313407) - - - - - 7d22504b by Zubin Duggal at 2026-03-04T12:15:44+05:30 Prepare release 9.12.4 - - - - - 21 changed files: - compiler/GHC/ByteCode/Asm.hs - compiler/GHC/Core/Opt/Simplify/Iteration.hs - compiler/GHC/Core/Opt/Simplify/Utils.hs - compiler/GHC/Core/Utils.hs - compiler/GHC/Tc/Instance/Class.hs - compiler/GHC/Tc/Solver.hs - compiler/GHC/Tc/Solver/Dict.hs - compiler/GHC/Tc/Types/Evidence.hs - compiler/GHC/Types/DefaultEnv.hs - + docs/users_guide/9.12.4-notes.rst - docs/users_guide/release-notes.rst - libraries/base/base.cabal.in - libraries/base/changelog.md - rts/Disassembler.c - rts/Interpreter.c - + testsuite/tests/default/T25775.hs - + testsuite/tests/default/T25775.stderr - testsuite/tests/default/all.T - + testsuite/tests/typecheck/should_compile/T26451.hs - testsuite/tests/typecheck/should_compile/all.T - testsuite/tests/typecheck/should_fail/T12921.stderr Changes: ===================================== compiler/GHC/ByteCode/Asm.hs ===================================== @@ -660,9 +660,22 @@ assembleI platform i = case i of tick_addr <- addr tick_mod info_addr <- addr info_mod np <- addr cc + let -- cast that checks that round-tripping through + -- Word32 doesn't change the value + toW32 x = let r = fromIntegral x :: Word32 + in if fromIntegral r == x + then r + else pprPanic "schemeER_wrk: breakpoint tick/info index too large!" (ppr x) + tick32 = toW32 tickx + tick_hi = fromIntegral (tick32 `shiftR` 16) + tick_lo = fromIntegral (tick32 .&. 0xffff) + info32 = toW32 infox + info_hi = fromIntegral (info32 `shiftR` 16) + info_lo = fromIntegral (info32 .&. 0xffff) emit bci_BRK_FUN [ Op p1 , Op tick_addr, Op info_addr - , SmallOp tickx, SmallOp infox + , SmallOp tick_hi, SmallOp tick_lo + , SmallOp info_hi, SmallOp info_lo , Op np ] ===================================== compiler/GHC/Core/Opt/Simplify/Iteration.hs ===================================== @@ -1731,6 +1731,7 @@ simplCast env body co0 cont0 , sc_hole_ty = coercionLKind co }) } -- NB! As the cast goes past, the -- type of the hole changes (#16312) + -- (f |> co) e ===> (f (e |> co1)) |> co2 -- where co :: (s1->s2) ~ (t1->t2) -- co1 :: t1 ~ s1 ===================================== compiler/GHC/Core/Opt/Simplify/Utils.hs ===================================== @@ -73,6 +73,7 @@ import GHC.Types.Tickish import GHC.Types.Demand import GHC.Types.Var.Set import GHC.Types.Basic +import GHC.Types.Name.Env import GHC.Data.OrdList ( isNilOL ) import GHC.Data.FastString ( fsLit ) @@ -82,9 +83,9 @@ import GHC.Utils.Monad import GHC.Utils.Outputable import GHC.Utils.Panic -import Control.Monad ( when ) +import Control.Monad ( guard, when ) import Data.List ( sortBy ) -import GHC.Types.Name.Env +import Data.Maybe import Data.Graph {- ********************************************************************* @@ -2471,7 +2472,27 @@ Note [Eliminate Identity Case] True -> True; False -> False -and similar friends. +and similar friends. There are some tricky wrinkles: + +(EIC1) Casts. We've seen this: + case e of x { _ -> x `cast` c } + And we definitely want to eliminate this case, to give + e `cast` c +(EIC2) Ticks. Similarly + case e of x { _ -> Tick t x } + At least if the tick is 'floatable' we want to eliminate the case + to give + Tick t e + +So `check_eq` strips off enclosing casts and ticks from the RHS of the +alternative, returning a wrapper function that will rebuild them around +the scrutinee if case-elim is successful. + +(EIC3) What if there are many alternatives, all identities. If casts + are involved they must be the same cast, to make the types line up. + In principle there could be different ticks in each RHS, but we just + pick the ticks from the first alternative. (In the common case there + is only one alternative.) Note [Scrutinee Constant Folding] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -2665,45 +2686,47 @@ mkCase mode scrut outer_bndr alts_ty alts -- See Note [Eliminate Identity Case] -------------------------------------------------- -mkCase1 _mode scrut case_bndr _ alts@(Alt _ _ rhs1 : alts') -- Identity case - | all identity_alt alts +mkCase1 _mode scrut case_bndr _ (alt1 : alts) -- Identity case + | Just wrap <- identity_alt alt1 -- `wrap`: see (EIC1) and (EIC2) + , all (isJust . identity_alt) alts -- See (EIC3) in Note [Eliminate Identity Case] = do { tick (CaseIdentity case_bndr) - ; return (mkTicks ticks $ re_cast scrut rhs1) } + ; return (wrap scrut) } where - ticks = concatMap (\(Alt _ _ rhs) -> stripTicksT tickishFloatable rhs) alts' - identity_alt (Alt con args rhs) = check_eq rhs con args - - check_eq (Cast rhs co) con args -- See Note [RHS casts] - = not (any (`elemVarSet` tyCoVarsOfCo co) args) && check_eq rhs con args - check_eq (Tick t e) alt args - = tickishFloatable t && check_eq e alt args - - check_eq (Lit lit) (LitAlt lit') _ = lit == lit' - check_eq (Var v) _ _ | v == case_bndr = True - check_eq (Var v) (DataAlt con) args - | null arg_tys, null args = v == dataConWorkId con - -- Optimisation only - check_eq rhs (DataAlt con) args = cheapEqExpr' tickishFloatable rhs $ - mkConApp2 con arg_tys args - check_eq _ _ _ = False + identity_alt :: CoreAlt -> Maybe (CoreExpr -> CoreExpr) + identity_alt (Alt con args rhs) = check_eq con args rhs + + check_eq :: AltCon -> [Var] -> CoreExpr -> Maybe (CoreExpr -> CoreExpr) + -- (check_eq con args e) return True if + -- e looks like (Tick (Cast (Tick (con args)))) + -- where (con args) is the LHS of the alternative + -- In that case it returns (\e. Tick (Cast (Tick e))), + -- a wrapper function that can rebuild the tick/cast stuff + -- See (EIC1) and (EIC2) in Note [Eliminate Identity Case] + check_eq alt_con args (Cast e co) -- See (EIC1) + = do { guard (not (any (`elemVarSet` tyCoVarsOfCo co) args)) + ; wrap <- check_eq alt_con args e + ; return (flip mkCast co . wrap) } + check_eq alt_con args (Tick t e) -- See (EIC2) + = do { guard (tickishFloatable t) + ; wrap <- check_eq alt_con args e + ; return (Tick t . wrap) } + check_eq alt_con args e + | is_id alt_con args e = Just (\e -> e) + | otherwise = Nothing + + is_id :: AltCon -> [Var] -> CoreExpr -> Bool + is_id _ _ (Var v) | v == case_bndr = True + is_id (LitAlt lit') _ (Lit lit) = lit == lit' + is_id (DataAlt con) args rhs + | Var v <- rhs -- Optimisation only + , null arg_tys + , null args = v == dataConWorkId con + | otherwise = cheapEqExpr' tickishFloatable rhs $ + mkConApp2 con arg_tys args + is_id _ _ _ = False arg_tys = tyConAppArgs (idType case_bndr) - -- Note [RHS casts] - -- ~~~~~~~~~~~~~~~~ - -- We've seen this: - -- case e of x { _ -> x `cast` c } - -- And we definitely want to eliminate this case, to give - -- e `cast` c - -- So we throw away the cast from the RHS, and reconstruct - -- it at the other end. All the RHS casts must be the same - -- if (all identity_alt alts) holds. - -- - -- Don't worry about nested casts, because the simplifier combines them - - re_cast scrut (Cast rhs co) = Cast (re_cast scrut rhs) co - re_cast scrut _ = scrut - mkCase1 mode scrut bndr alts_ty alts = mkCase2 mode scrut bndr alts_ty alts ===================================== compiler/GHC/Core/Utils.hs ===================================== @@ -251,7 +251,7 @@ applyTypeToArgs pp_e op_ty args mkCastMCo :: CoreExpr -> MCoercionR -> CoreExpr mkCastMCo e MRefl = e -mkCastMCo e (MCo co) = Cast e co +mkCastMCo e (MCo co) = mkCast e co -- We are careful to use (MCo co) only when co is not reflexive -- Hence (Cast e co) rather than (mkCast e co) @@ -302,40 +302,41 @@ mkCast expr co -- | Wraps the given expression in the source annotation, dropping the -- annotation if possible. mkTick :: CoreTickish -> CoreExpr -> CoreExpr -mkTick t orig_expr = mkTick' id id orig_expr +mkTick t orig_expr = mkTick' id orig_expr where -- Some ticks (cost-centres) can be split in two, with the -- non-counting part having laxer placement properties. canSplit = tickishCanSplit t && tickishPlace (mkNoCount t) /= tickishPlace t + -- mkTick' handles floating of ticks *into* the expression. - -- In this function, `top` is applied after adding the tick, and `rest` before. - -- This will result in applications that look like (top $ Tick t $ rest expr). - -- If we want to push the tick deeper, we pre-compose `top` with a function - -- adding the tick. - mkTick' :: (CoreExpr -> CoreExpr) -- apply after adding tick (float through) - -> (CoreExpr -> CoreExpr) -- apply before adding tick (float with) - -> CoreExpr -- current expression + mkTick' :: (CoreExpr -> CoreExpr) -- Apply before adding tick (float with) + -- Always a composition of (Tick t) wrappers + -> CoreExpr -- Current expression -> CoreExpr - mkTick' top rest expr = case expr of + -- So in the call (mkTick' rest e), the expression + -- (rest e) + -- has the same type as e + -- Returns an expression equivalent to (Tick t (rest e)) + mkTick' rest expr = case expr of -- Float ticks into unsafe coerce the same way we would do with a cast. Case scrut bndr ty alts@[Alt ac abs _rhs] | Just rhs <- isUnsafeEqualityCase scrut bndr alts - -> top $ mkTick' (\e -> Case scrut bndr ty [Alt ac abs e]) rest rhs + -> Case scrut bndr ty [Alt ac abs (mkTick' rest rhs)] -- Cost centre ticks should never be reordered relative to each -- other. Therefore we can stop whenever two collide. Tick t2 e - | ProfNote{} <- t2, ProfNote{} <- t -> top $ Tick t $ rest expr + | ProfNote{} <- t2, ProfNote{} <- t -> Tick t $ rest expr -- Otherwise we assume that ticks of different placements float -- through each other. - | tickishPlace t2 /= tickishPlace t -> mkTick' (top . Tick t2) rest e + | tickishPlace t2 /= tickishPlace t -> Tick t2 $ mkTick' rest e -- For annotations this is where we make sure to not introduce -- redundant ticks. - | tickishContains t t2 -> mkTick' top rest e - | tickishContains t2 t -> orig_expr - | otherwise -> mkTick' top (rest . Tick t2) e + | tickishContains t t2 -> mkTick' rest e -- Drop t2 + | tickishContains t2 t -> rest e -- Drop t + | otherwise -> mkTick' (rest . Tick t2) e -- Ticks don't care about types, so we just float all ticks -- through them. Note that it's not enough to check for these @@ -343,14 +344,14 @@ mkTick t orig_expr = mkTick' id id orig_expr -- expressions below ticks, such constructs can be the result of -- unfoldings. We therefore make an effort to put everything into -- the right place no matter what we start with. - Cast e co -> mkTick' (top . flip Cast co) rest e - Coercion co -> Coercion co + Cast e co -> mkCast (mkTick' rest e) co + Coercion co -> Tick t $ rest (Coercion co) Lam x e -- Always float through type lambdas. Even for non-type lambdas, -- floating is allowed for all but the most strict placement rule. | not (isRuntimeVar x) || tickishPlace t /= PlaceRuntime - -> mkTick' (top . Lam x) rest e + -> Lam x $ mkTick' rest e -- If it is both counting and scoped, we split the tick into its -- two components, often allowing us to keep the counting tick on @@ -359,25 +360,25 @@ mkTick t orig_expr = mkTick' id id orig_expr -- floated, and the lambda may then be in a position to be -- beta-reduced. | canSplit - -> top $ Tick (mkNoScope t) $ rest $ Lam x $ mkTick (mkNoCount t) e + -> Tick (mkNoScope t) $ rest $ Lam x $ mkTick (mkNoCount t) e App f arg -- Always float through type applications. | not (isRuntimeArg arg) - -> mkTick' (top . flip App arg) rest f + -> App (mkTick' rest f) arg -- We can also float through constructor applications, placement -- permitting. Again we can split. | isSaturatedConApp expr && (tickishPlace t==PlaceCostCentre || canSplit) -> if tickishPlace t == PlaceCostCentre - then top $ rest $ tickHNFArgs t expr - else top $ Tick (mkNoScope t) $ rest $ tickHNFArgs (mkNoCount t) expr + then rest $ tickHNFArgs t expr + else Tick (mkNoScope t) $ rest $ tickHNFArgs (mkNoCount t) expr Var x | notFunction && tickishPlace t == PlaceCostCentre - -> orig_expr + -> rest expr -- Drop t | notFunction && canSplit - -> top $ Tick (mkNoScope t) $ rest expr + -> Tick (mkNoScope t) $ rest expr where -- SCCs can be eliminated on variables provided the variable -- is not a function. In these cases the SCC makes no difference: @@ -389,10 +390,10 @@ mkTick t orig_expr = mkTick' id id orig_expr Lit{} | tickishPlace t == PlaceCostCentre - -> orig_expr + -> rest expr -- Drop t -- Catch-all: Annotate where we stand - _any -> top $ Tick t $ rest expr + _any -> Tick t $ rest expr mkTicks :: [CoreTickish] -> CoreExpr -> CoreExpr mkTicks ticks expr = foldr mkTick expr ticks ===================================== compiler/GHC/Tc/Instance/Class.hs ===================================== @@ -47,6 +47,7 @@ import GHC.Core.Make ( mkCharExpr, mkNaturalExpr, mkStringExprFS, mkCoreLams ) import GHC.Core.DataCon import GHC.Core.TyCon import GHC.Core.Class +import GHC.Core.Utils( mkCast ) import GHC.Core ( Expr(..) ) @@ -456,7 +457,7 @@ matchWithDict [cls, mty] mkCoreLams [ runtimeRep1TyVar, openAlphaTyVar, sv, k ] $ Var k `App` - (Var sv `Cast` mkTransCo (mkSubCo co2) (mkSymCo co)) + (Var sv `mkCast` mkTransCo (mkSubCo co2) (mkSymCo co)) ; tc <- tcLookupTyCon withDictClassName ; let Just withdict_data_con @@ -935,7 +936,7 @@ matchDataToTag dataToTagClass [levity, dty] = do dataToTagDataCon = tyConSingleDataCon (classTyCon dataToTagClass) mk_ev _ = evDataConApp dataToTagDataCon [levity, dty] - [methodRep `Cast` methodCo] + [methodRep `mkCast` methodCo] -> addUsedDataCons rdr_env repTyCon -- See wrinkles DTW2 and DTW3 $> OneInst { cir_new_theta = [] -- (Ignore stupid theta.) , cir_mk_ev = mk_ev ===================================== compiler/GHC/Tc/Solver.hs ===================================== @@ -93,7 +93,7 @@ import Control.Monad import Control.Monad.Trans.Class ( lift ) import Control.Monad.Trans.State.Strict ( StateT(runStateT), put ) import Data.Foldable ( toList, traverse_ ) -import Data.List ( partition, intersect ) +import Data.List ( partition ) import Data.List.NonEmpty ( NonEmpty(..), nonEmpty ) import qualified Data.List.NonEmpty as NE import GHC.Data.Maybe ( isJust, mapMaybe, catMaybes ) @@ -3755,32 +3755,74 @@ Type-class defaulting deals with the situation where we have unsolved constraints like (Num alpha), where `alpha` is a unification variable. We want to pick a default for `alpha`, such as `alpha := Int` to resolve the ambiguity. -Type-class defaulting is guided by the `DefaultEnv`: see Note [Named default declarations] -in GHC.Tc.Gen.Default +The function 'tryTypeClassDefaulting' implements type-class defaulting. The +algorithm for defaulting depends on whether certain extensions are enabled, +such as -XOverloadedStrings or -XExtendedDefaultRules. To explain this, let us +define the following: -The entry point for defaulting the unsolved constraints is `applyDefaultingRules`, -which depends on `disambigGroup`, which in turn depends on workhorse -`disambigProposalSequences`. The latter is also used by defaulting plugins through -`disambigMultiGroup` (see Note [Defaulting plugins] below). + Unary typeclass: + a typeclass with a single visible type argument. -The algorithm works as follows. Let S be the complete set of unsolved -constraints, and initialize Sx to an empty set of constraints. For every type -variable `v` that is free in S: + Examples: -1. Define Cv = { Ci v | Ci v ∈ S }, the subset of S consisting of all constraints in S of - form (Ci v), where Ci is a single-parameter type class. (We do no defaulting for - multi-parameter type classes.) + Num :: Type -> Constraint + Eq :: Type -> Constraint + Foldable :: (Type -> Type) -> Constraint + Typeable :: forall k. k -> Constraint -- NB: also has an /invisible/ argument -2. Define Dv, by extending Cv with the superclasses of every Ci in Cv + Non-examples: -3. Define Ev, by filtering Dv to contain only classes with a default declaration. + Nullary :: Constraint + Binary :: Type -> Type -> Constraint + Binary2 :: forall k -> k -> Constraint -- Two visible arguments -4. For each Ci in Ev, if Ci has a non-empty default list in the `DefaultEnv`, find the first - type T in the default list for Ci for which, for every (Ci v) in Cv, the constraint (Ci T) - is soluble. + Defaultable class + a typeclass which has at least one in-scope default declaration -5. If there is precisely one type T in the resulting type set, resolve the ambiguity by adding - a constraint (v~ Ti) constraint to a set Sx; otherwise report a static error. + This includes the two different categories of default declarations: + + - Haskell 98 default declarations such as 'default (Integer, Float)'. + + - `Num` is always defaultable; either the user says 'default( Integer, Float )' + or (absent such a declaration) the system fills in a fallback default declaration. + See Section 4.3.4 in https://www.haskell.org/onlinereport/haskell2010/haskellch4.html + + - With `OverloadedStrings`, the class `IsString` is defaultable + - With `ExtendedDefaultRules`, the classes `Show`, `Eq`, `Ord`, `Foldable` and `Traversable` + are defaultable + + - Named default declarations, which apply to the named class, e.g. + 'default Cls(X, Y)' applies precisely to 'Cls'. + Note that these may be locally defined, or they may be imported. + + Standard class: + a class defined in the Prelude or the standard library, as defined + by the Haskell 98 report (section 4.3.4) + + These are defined in GHC.Builtin.Names.standardClassKeys. + +The rules for defaulting a collection 'S' of unsolved constraints are as follows: + + 1. For each metavariable 'v' appearing in 'S', define + + U_v = { C v | C v ∈ U, C is a unary typeclass } + + We then process each 'U_v' in turn, in order to find a defaulting + assignment 'v := ty' that solves all of 'U_v'. + + 2. Unless -XExtendedDefaultRules is in effect, give up if 'v' appears: + + - in any constraint that isn't a unary class constraint + - in a class constraint which is non-standard and does not have + a default declaration in scope. + + 3. Compute candidate assignments: for each unary typeclass 'C' in 'U_v' which + has a default declaration in scope, find the first type 'ty' in the list + of in-scope default types for 'C' for which all of 'U_v' is soluble. + + 4. If there is precisely one type candidate type assignment 'ty' that allows + all of 'U_v' to be solved, we default 'v := ty'. Otherwise, do nothing + ('v' remains ambiguous). Note [Defaulting plugins] ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -3919,8 +3961,8 @@ findDefaultableGroups (default_tys, extended_defaults) wanteds -- Finds unary type-class constraints -- But take account of polykinded classes like Typeable, - -- which may look like (Typeable * (a:*)) (#8931) - -- step (1) in Note [How type-class constraints are defaulted] + -- which may look like (Typeable Type (a:Type)) (#8931) + -- See step (1) in Note [How type-class constraints are defaulted] find_unary :: Ct -> Either (Ct, Class, TyVar) Ct find_unary cc | Just (cls,tys) <- getClassPredTys_maybe (ctPred cc) @@ -3932,21 +3974,42 @@ findDefaultableGroups (default_tys, extended_defaults) wanteds = Left (cc, cls, tv) find_unary cc = Right cc -- Non unary or non dictionary - bad_tvs :: TcTyCoVarSet -- TyVars mentioned by non-unaries - bad_tvs = mapUnionVarSet tyCoVarsOfCt non_unaries + nonunary_tvs :: TcTyCoVarSet -- TyVars mentioned by non-unaries + nonunary_tvs = mapUnionVarSet tyCoVarsOfCt non_unaries cmp_tv (_,_,tv1) (_,_,tv2) = tv1 `compare` tv2 defaultable_tyvar :: TcTyVar -> Bool defaultable_tyvar tv = let b1 = isTyConableTyVar tv -- Note [Avoiding spurious errors] - b2 = not (tv `elemVarSet` bad_tvs) + b2 = not (tv `elemVarSet` nonunary_tvs) in b1 && (b2 || extended_defaults) -- Note [Multi-parameter defaults] - -- Determines if any of the given type class constructors is in default_tys - -- step (3) in Note [How type-class constraints are defaulted] + -- Determines whether the collection of class constraints permits defaulting. + -- See step (2) in Note [How type-class constraints are defaulted] defaultable_classes :: [Class] -> Bool - defaultable_classes clss = not . null . intersect clss $ map cd_class default_tys + defaultable_classes clss = + -- One of the classes has a default declaration in scope + -- (this includes 'Num', and e.g. 'IsString' with -XOverloadedStrings) + any (`elementOfUniqSet` classes_with_defaults) clss + && + -- AND, either: + -- - ExtendedDefaultRules is in effect, or + -- - all the classes are standard or have a default declaration in scope + (extended_defaults || all is_std_or_has_default clss) + is_std_or_has_default :: Class -> Bool + is_std_or_has_default cls = + (getUnique cls `elem` standardClassKeys) + || + (cls `elementOfUniqSet` classes_with_defaults) + + -- All classes with a default declaration in scope; either: + -- + -- - a named default declaration such as 'default C(Double, Bool)', or + -- - a Haskell 98 default declaration such as 'default(Int, Float)', + -- which adds defaults for Num, for IsString with OverloadedStrings, + -- and for Foldable/Traversable/... with ExtendedDefaultRules + classes_with_defaults = mkUniqSet $ map cd_class default_tys ------------------------------ @@ -3996,14 +4059,14 @@ disambigProposalSequences orig_wanteds wanteds proposalSequences allConsistent = do { traverse_ (traverse_ reportInvalidDefaultedTyVars . getProposalSequence) proposalSequences ; fake_ev_binds_var <- TcS.newTcEvBinds ; tclvl <- TcS.getTcLevel - -- Step (4) in Note [How type-class constraints are defaulted] + -- Step (3) in Note [How type-class constraints are defaulted] ; successes <- fmap catMaybes $ nestImplicTcS fake_ev_binds_var (pushTcLevel tclvl) $ mapM firstSuccess proposalSequences ; traceTcS "disambigProposalSequences" (vcat [ ppr wanteds , ppr proposalSequences , ppr successes ]) - -- Step (5) in Note [How type-class constraints are defaulted] + -- Step (4) in Note [How type-class constraints are defaulted] ; case successes of success@(tvs, subst) : rest | allConsistent (success :| rest) ===================================== compiler/GHC/Tc/Solver/Dict.hs ===================================== @@ -102,28 +102,53 @@ solveDict dict_ct@(DictCt { di_ev = ev, di_cls = cls, di_tys = tys }) ; stopWithStage (dictCtEvidence dict_ct) "Kept inert DictCt" } updInertDicts :: DictCt -> TcS () -updInertDicts dict_ct@(DictCt { di_cls = cls, di_ev = ev, di_tys = tys }) - = do { traceTcS "Adding inert dict" (ppr dict_ct $$ ppr cls <+> ppr tys) - - ; if | isGiven ev, Just (str_ty, _) <- isIPPred_maybe cls tys - -> -- See (SIP1) and (SIP2) in Note [Shadowing of implicit parameters] - -- Update /both/ inert_cans /and/ inert_solved_dicts. - updInertSet $ \ inerts@(IS { inert_cans = ics, inert_solved_dicts = solved }) -> - inerts { inert_cans = updDicts (filterDicts (does_not_mention_ip_for str_ty)) ics - , inert_solved_dicts = filterDicts (does_not_mention_ip_for str_ty) solved } - | otherwise - -> return () +updInertDicts dict_ct + = do { traceTcS "Adding inert dict" (ppr dict_ct) + + -- For Given implicit parameters (only), delete any existing + -- Givens for the same implicit parameter. + -- See Note [Shadowing of implicit parameters] + ; deleteGivenIPs dict_ct -- Add the new constraint to the inert set ; updInertCans (updDicts (addDict dict_ct)) } + +deleteGivenIPs :: DictCt -> TcS () +-- Special magic when adding a Given implicit parameter to the inert set +-- For [G] ?x::ty, remove any existing /Givens/ mentioning ?x, +-- from /both/ inert_cans /and/ inert_solved_dicts (#23761) +-- See Note [Shadowing of implicit parameters] +deleteGivenIPs (DictCt { di_cls = cls, di_ev = ev, di_tys = tys }) + | isGiven ev + , Just (str_ty, _) <- isIPPred_maybe cls tys + = updInertSet $ \ inerts@(IS { inert_cans = ics, inert_solved_dicts = solved }) -> + inerts { inert_cans = updDicts (filterDicts (keep_can str_ty)) ics + , inert_solved_dicts = filterDicts (keep_solved str_ty) solved } + | otherwise + = return () where - -- Does this class constraint or any of its superclasses mention - -- an implicit parameter (?str :: ty) for the given 'str' and any type 'ty'? - does_not_mention_ip_for :: Type -> DictCt -> Bool - does_not_mention_ip_for str_ty (DictCt { di_cls = cls, di_tys = tys }) - = not $ mentionsIP (not . typesAreApart str_ty) (const True) cls tys - -- See Note [Using typesAreApart when calling mentionsIP] - -- in GHC.Core.Predicate + keep_can, keep_solved :: Type -> DictCt -> Bool + -- keep_can: we keep an inert dictionary UNLESS + -- (1) it is a Given + -- (2) it binds an implicit parameter (?str :: ty) for the given 'str' + -- regardless of 'ty', possibly via its superclasses + -- The test is a bit conservative, hence `mentionsIP` and `typesAreApart` + -- See Note [Using typesAreApart when calling mentionsIP] + -- in GHC.Core.Predicate + -- + -- keep_solved: same as keep_can, but for /all/ constraints not just Givens + -- + -- Why two functions? See (SIP3) in Note [Shadowing of implicit parameters] + keep_can str (DictCt { di_ev = ev, di_cls = cls, di_tys = tys }) + = not (isGiven ev -- (1) + && mentions_ip str cls tys) -- (2) + keep_solved str (DictCt { di_cls = cls, di_tys = tys }) + = not (mentions_ip str cls tys) + + -- mentions_ip: the inert constraint might provide evidence + -- for an implicit parameter (?str :: ty) for the given 'str' + mentions_ip str cls tys + = mentionsIP (not . typesAreApart str) (const True) cls tys canDictCt :: CtEvidence -> Class -> [Type] -> SolverStage DictCt -- Once-only processing of Dict constraints: @@ -220,7 +245,9 @@ in two places: * In `updInertDicts`, in this module, when adding [G] (?x :: ty), remove any existing [G] (?x :: ty'), regardless of ty'. -* Wrinkle (SIP1): we must be careful of superclasses. Consider +There are wrinkles: + +* Wrinkle (SIP1): we must be careful of superclasses (#14218). Consider f,g :: (?x::Int, C a) => a -> a f v = let ?x = 4 in g v @@ -228,24 +255,31 @@ in two places: We must /not/ solve this from the Given (?x::Int, C a), because of the intervening binding for (?x::Int). #14218. - We deal with this by arranging that when we add [G] (?x::ty) we delete + We deal with this by arranging that when we add [G] (?x::ty) we /delete/ * from the inert_cans, and * from the inert_solved_dicts any existing [G] (?x::ty) /and/ any [G] D tys, where (D tys) has a superclass with (?x::ty). See Note [Local implicit parameters] in GHC.Core.Predicate. - An important special case is constraint tuples like [G] (% ?x::ty, Eq a %). - But it could happen for `class xx => D xx where ...` and the constraint D - (?x :: int). This corner (constraint-kinded variables instantiated with - implicit parameter constraints) is not well explored. + An very important special case is constraint tuples like [G] (% ?x::ty, Eq a %). + + But it could also happen for `class xx => D xx where ...` and the constraint + D (?x :: int); again see Note [Local implicit parameters]. This corner + (constraint-kinded variables instantiated with implicit parameter constraints) + is not well explored. - Example in #14218, and #23761 + You might worry about whether deleting an /entire/ constraint just because + a distant superclass has an implicit parameter might make another Wanted for + that constraint un-solvable. Indeed so. But for constraint tuples it doesn't + matter -- their entire payload is their superclasses. And the other case is + the ill-explored corner above. The code that accounts for (SIP1) is in updInertDicts; in particular the call to GHC.Core.Predicate.mentionsIP. * Wrinkle (SIP2): we must apply this update semantics for `inert_solved_dicts` - as well as `inert_cans`. + as well as `inert_cans` (#23761). + You might think that wouldn't be necessary, because an element of `inert_solved_dicts` is never an implicit parameter (see Note [Solved dictionaries] in GHC.Tc.Solver.InertSet). @@ -258,6 +292,19 @@ in two places: Now (C (?x::Int)) has a superclass (?x::Int). This may look exotic, but it happens particularly for constraint tuples, like `(% ?x::Int, Eq a %)`. +* Wrinkle (SIP3) + - Note that for the inert dictionaries, `inert_cans`, we must /only/ delete + existing /Givens/! Deleting an existing Wanted led to #26451; we just never + solved it! + + - In contrast, the solved dictionaries, `inert_solved_dicts`, are really like + Givens; they may be "inherited" from outer scopes, so we must delete any + solved dictionaries for this implicit parameter for /both/ Givens /and/ + Wanteds. + + Otherwise the new Given doesn't properly shadow those inherited solved + dictionaries. Test T23761 showed this up. + Example 1: Suppose we have (typecheck/should_compile/ImplicitParamFDs) ===================================== compiler/GHC/Tc/Types/Evidence.hs ===================================== @@ -56,6 +56,7 @@ import GHC.Types.Var import GHC.Types.Id( idScaledType ) import GHC.Core.Coercion.Axiom import GHC.Core.Coercion +import GHC.Core.Utils( mkCast ) import GHC.Core.Ppr () -- Instance OutputableBndr TyVar import GHC.Tc.Utils.TcType import GHC.Core.Type @@ -528,7 +529,7 @@ evCoercion co = EvExpr (Coercion co) -- | d |> co evCast :: EvExpr -> TcCoercion -> EvTerm evCast et tc | isReflCo tc = EvExpr et - | otherwise = EvExpr (Cast et tc) + | otherwise = EvExpr (mkCast et tc) -- Dictionary instance application evDFunApp :: DFunId -> [Type] -> [EvExpr] -> EvTerm ===================================== compiler/GHC/Types/DefaultEnv.hs ===================================== @@ -31,9 +31,13 @@ import Data.List (sortBy) import Data.Function (on) -- See Note [Named default declarations] in GHC.Tc.Gen.Default + -- | Default environment mapping class name @Name@ to their default type lists +-- +-- NB: this includes Haskell98 default declarations, at the 'Num' key. type DefaultEnv = NameEnv ClassDefaults +-- | Defaulting type assignments for the given class. data ClassDefaults = ClassDefaults { cd_class :: Class -- ^ The class whose defaults are being defined , cd_types :: [Type] ===================================== docs/users_guide/9.12.4-notes.rst ===================================== @@ -0,0 +1,154 @@ +.. _release-9-12-4: + +Version 9.12.4 +============== + +The significant changes to the various parts of the compiler are listed in the +following sections. See the `migration guide +<https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.12>`_ on the GHC Wiki +for specific guidance on migrating programs to this release. + +Compiler +~~~~~~~~ + +- Fixed a bug in CSE where the in-scope set was not properly maintained (:ghc-ticket:`25468`) +- Fixed ``matchExpectedFunTys`` to use ``tcMkScaledFunTys`` (:ghc-ticket:`26277`) +- Fixed ``parenBreakableList`` usage in ``ppHsContext`` for better pretty-printing of contexts +- Improved error messages for bad record updates to allow out-of-scope data constructors (:ghc-ticket:`26391`) +- Fixed a missing InVar->OutVar lookup in ``SetLevels`` (:ghc-ticket:`26681`) +- Fixed split sections on Windows (:ghc-ticket:`26696`, :ghc-ticket:`26494`) +- Fixed split sections for the LLVM backend (:ghc-ticket:`26770`) +- Don't re-use stack slots for growing registers (:ghc-ticket:`26668`) +- Fixed cast worker/wrapper incorrectly firing on INLINE functions (:ghc-ticket:`26903`) +- Fixed non-determinism in ``TyLitMap`` by using deterministic maps for strings (:ghc-ticket:`26846`) +- Fixed non-determinism in ``WithHsDocIdentifiers`` binary instance by using a stable sort (:ghc-ticket:`26858`) +- Added ``-mcmodel=medium`` module flag to generated LLVM IR on LoongArch +- Pass the ``mcmodel=medium`` parameter to CC via GHC on LoongArch +- Pass the ``+evex512`` attribute to LLVM 18+ when ``-mavx512f`` is set (:ghc-ticket:`26410`) +- Improved error handling in ``getPackageArchives`` (:ghc-ticket:`26383`) +- Fixed a shadowing bug in implicit parameters (:ghc-ticket:`26451`) +- Fixed a subtle bug in ``GHC.Core.Utils.mkTick`` that could generate type-incorrect code (:ghc-ticket:`26772`) +- Fixed a long-standing interaction between ticks and casts in ``Eliminate Identity Cases`` +- ``NamedDefaults``: require the class to be standard or have an in-scope default declaration (:ghc-ticket:`25775`, :ghc-ticket:`25778`) + +Runtime System +~~~~~~~~~~~~~~ + +- Fixed a deadlock with eventlog flush interval and RTS shutdown (:ghc-ticket:`26573`) +- Fixed eager black holes: record mutated closure and fix assertion (:ghc-ticket:`26495`) +- Fixed object file format detection in ``loadArchive`` (:ghc-ticket:`26630`) +- Use ``INFO_TABLE_CONSTR`` for ``stg_dummy_ret_closure`` (:ghc-ticket:`26745`) +- Fixed lost wakeups in ``threadPaused`` for threads blocked on black holes (:ghc-ticket:`26324`) +- Fixed ``stg_AP_STACK`` to push the correct update frame (:ghc-ticket:`26324`) +- Fixed potential loop in heap reservation logic on certain kernels (:ghc-ticket:`26151`) +- Don't use CAS without ``PARALLEL_GC`` on +- Switch prim to use modern atomic compiler builtins (:ghc-ticket:`26729`) +- Removed obsolete ``CC_SUPPORTS_TLS``, ``HAS_VISIBILITY_HIDDEN``, ``COMPILING_WINDOWS_DLL``, + and ``__GNUC__``-related logic +- Removed the ``-O3`` pragma hack in ``Hash.c`` +- Removed unnecessary Cabal flags + +Code Generation +~~~~~~~~~~~~~~~ + +- NCG for PPC: add pattern for ``CmmRegOff`` to ``iselExpr64`` (:ghc-ticket:`26828`) +- PPC NCG: Use libcall for 64-bit ``cmpxchg`` on 32-bit PowerPC (:ghc-ticket:`23969`) + +Bytecode Compiler +~~~~~~~~~~~~~~~~~ + +- Use 32 bits for breakpoint index (:ghc-ticket:`26325`) + +``base`` library +~~~~~~~~~~~~~~~~ + +- Expose ``Backtraces`` constructor and fields (:ghc-ticket:`26049`) +- Store ``StackTrace`` and ``StackSnapshot`` in ``Backtraces``, deferring decoding until display +- Evaluate backtraces for ``error`` exceptions at the moment they are thrown + (`CLC proposal #383 <https://github.com/haskell/core-libraries-committee/issues/383>`__, + :ghc-ticket:`26751`) + +``ghc-experimental`` library +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +- Fixed ``GHC.Exception.Backtrace.Experimental`` module +- Added ability to customise the collection of exception annotations + +``ghc-pkg`` +~~~~~~~~~~~~ + +- Removed ``traceId`` from ``ghc-pkg`` executable + +``ghc-toolchain`` +~~~~~~~~~~~~~~~~~ + +- Dropped ``ld.gold`` from merge object command + +Build System +~~~~~~~~~~~~ + +- Added ``hpc`` to release script +- Use a response file to invoke GHC when analysing dependencies +- Fixed ``GHC.Platform.Host`` generation for cross stage1 (:ghc-ticket:`26449`) +- Fixed runtime error during ``users_guide`` build with Sphinx 9.1.0 (:ghc-ticket:`26810`) +- Added ``ghc-{experimental,internal}.cabal`` to the list of dependencies of the doc target (:ghc-ticket:`26738`) + +Wasm Backend +~~~~~~~~~~~~ + +- Fixed dyld handling for forward declared ``GOT.func`` items (:ghc-ticket:`26430`) +- Ensure ``setKeepCAFs()`` is called in GHCi (:ghc-ticket:`26106`) +- Prevent bundlers from resolving ``import("node:timers")`` +- Use ``import.meta.main`` for proper distinction of Node.js main modules (:ghc-ticket:`26916`) + +Included libraries +~~~~~~~~~~~~~~~~~~ + +The package database provided with this distribution also contains a number of +packages other than GHC itself. See the changelogs provided with these packages +for further change information. + +.. ghc-package-list:: + + compiler/ghc.cabal: The compiler itself + libraries/array/array.cabal: Dependency of ``ghc`` library + libraries/base/base.cabal: Core library + libraries/binary/binary.cabal: Dependency of ``ghc`` library + libraries/bytestring/bytestring.cabal: Dependency of ``ghc`` library + libraries/Cabal/Cabal/Cabal.cabal: Dependency of ``ghc-pkg`` utility + libraries/Cabal/Cabal-syntax/Cabal-syntax.cabal: Dependency of ``ghc-pkg`` utility + libraries/containers/containers/containers.cabal: Dependency of ``ghc`` library + libraries/deepseq/deepseq.cabal: Dependency of ``ghc`` library + libraries/directory/directory.cabal: Dependency of ``ghc`` library + libraries/exceptions/exceptions.cabal: Dependency of ``ghc`` and ``haskeline`` library + libraries/file-io/file-io.cabal: Dependency of ``directory`` library + libraries/filepath/filepath.cabal: Dependency of ``ghc`` library + libraries/ghc-boot/ghc-boot.cabal: Internal compiler library + libraries/ghc-boot-th/ghc-boot-th.cabal: Internal compiler library + libraries/ghc-compact/ghc-compact.cabal: Core library + libraries/ghc-experimental/ghc-experimental.cabal: Core library + libraries/ghc-heap/ghc-heap.cabal: GHC heap-walking library + libraries/ghci/ghci.cabal: The REPL interface + libraries/ghc-internal/ghc-internal.cabal: Core library + libraries/ghc-platform/ghc-platform.cabal: Internal library + libraries/ghc-prim/ghc-prim.cabal: Core library + libraries/haskeline/haskeline.cabal: Dependency of ``ghci`` executable + libraries/hpc/hpc.cabal: Dependency of ``hpc`` executable + libraries/integer-gmp/integer-gmp.cabal: Core library + libraries/mtl/mtl.cabal: Dependency of ``Cabal`` library + libraries/os-string/os-string.cabal: Dependency of ``filepath`` library + libraries/parsec/parsec.cabal: Dependency of ``Cabal`` library + libraries/pretty/pretty.cabal: Dependency of ``ghc`` library + libraries/process/process.cabal: Dependency of ``ghc`` library + libraries/semaphore-compat/semaphore-compat.cabal: Dependency of ``ghc`` library + libraries/stm/stm.cabal: Dependency of ``haskeline`` library + libraries/template-haskell/template-haskell.cabal: Core library + libraries/terminfo/terminfo.cabal: Dependency of ``haskeline`` library + libraries/text/text.cabal: Dependency of ``Cabal`` library + libraries/time/time.cabal: Dependency of ``ghc`` library + libraries/transformers/transformers.cabal: Dependency of ``ghc`` library + libraries/unix/unix.cabal: Dependency of ``ghc`` library + libraries/Win32/Win32.cabal: Dependency of ``ghc`` library + libraries/xhtml/xhtml.cabal: Dependency of ``haddock`` executable + utils/haddock/haddock-api/haddock-api.cabal: Dependency of ``haddock`` executable + utils/haddock/haddock-library/haddock-library.cabal: Dependency of ``haddock`` executable ===================================== docs/users_guide/release-notes.rst ===================================== @@ -7,3 +7,4 @@ Release notes 9.12.1-notes 9.12.2-notes 9.12.3-notes + 9.12.4-notes ===================================== libraries/base/base.cabal.in ===================================== @@ -4,7 +4,7 @@ cabal-version: 3.0 -- Make sure you are editing ghc-experimental.cabal.in, not ghc-experimental.cabal name: base -version: 4.21.1.0 +version: 4.21.2.0 -- NOTE: Don't forget to update ./changelog.md license: BSD-3-Clause ===================================== libraries/base/changelog.md ===================================== @@ -1,7 +1,9 @@ # Changelog for [`base` package](http://hackage.haskell.org/package/base) -## 4.21.2.0 *TBA* - * Evaluate backtraces for "error" exceptions at the moment they are thrown. ([CLC proposal #383](https://github.com/haskell/core-libraries-committee/issues/383)) +## 4.21.2.0 *March 2026* + * Expose `Backtraces` constructor and fields ([CLC proposal #199](https://github.com/haskell/core-libraries-committee/issues/199), [#26049](https://gitlab.haskell.org/ghc/ghc/-/issues/26049)) + * Store `StackTrace` and `StackSnapshot` in `Backtraces`, deferring decoding until display + * Evaluate backtraces for "error" exceptions at the moment they are thrown ([CLC proposal #383](https://github.com/haskell/core-libraries-committee/issues/383), [#26751](https://gitlab.haskell.org/ghc/ghc/-/issues/26751)) ## 4.21.1.0 *Sept 2024* * Fix incorrect results of `integerPowMod` when the base is 0 and the exponent is negative, and `integerRecipMod` when the modulus is zero ([#26017](https://gitlab.haskell.org/ghc/ghc/-/issues/26017)). ===================================== rts/Disassembler.c ===================================== @@ -87,12 +87,12 @@ disInstr ( StgBCO *bco, int pc ) case bci_BRK_FUN: debugBelch ("BRK_FUN " ); printPtr( ptrs[instrs[pc]] ); debugBelch (" %d ", instrs[pc+1]); printPtr( ptrs[instrs[pc+2]] ); - CostCentre* cc = (CostCentre*)literals[instrs[pc+5]]; + CostCentre* cc = (CostCentre*)literals[instrs[pc+7]]; if (cc) { debugBelch(" %s", cc->label); } debugBelch("\n"); - pc += 6; + pc += 8; break; case bci_SWIZZLE: { W_ stkoff = BCO_GET_LARGE_ARG; ===================================== rts/Interpreter.c ===================================== @@ -1286,8 +1286,8 @@ run_BCO: arg1_brk_array = BCO_GET_LARGE_ARG; arg2_tick_mod = BCO_GET_LARGE_ARG; arg3_info_mod = BCO_GET_LARGE_ARG; - arg4_tick_index = BCO_NEXT; - arg5_info_index = BCO_NEXT; + arg4_tick_index = BCO_READ_NEXT_32; + arg5_info_index = BCO_READ_NEXT_32; #if defined(PROFILING) arg6_cc = BCO_GET_LARGE_ARG; #else ===================================== testsuite/tests/default/T25775.hs ===================================== @@ -0,0 +1,19 @@ + + +module T25775 where + +import Data.Kind + +default (Int) + +type NonStd :: Type -> Constraint +class NonStd a where + +f :: (Num a, NonStd a) => a -> a +f = (+1) + +x :: String +x = show (f 0) + -- We should NOT default 0 to type Int, despite the top-level default + -- declaration in this module, because of the presence of the + -- non-standard class 'NonStd'. ===================================== testsuite/tests/default/T25775.stderr ===================================== @@ -0,0 +1,19 @@ +T25775.hs:16:5: error: [GHC-39999] + • Ambiguous type variable ‘a0’ arising from a use of ‘show’ + prevents the constraint ‘(Show a0)’ from being solved. + Probable fix: use a type annotation to specify what ‘a0’ should be. + Potentially matching instances: + instance Show Ordering -- Defined in ‘GHC.Internal.Show’ + instance Show Integer -- Defined in ‘GHC.Internal.Show’ + ...plus 25 others + ...plus 13 instances involving out-of-scope types + (use -fprint-potential-instances to see them all) + • In the expression: show (f 0) + In an equation for ‘x’: x = show (f 0) + +T25775.hs:16:11: error: [GHC-39999] + • No instance for ‘NonStd a0’ arising from a use of ‘f’ + • In the first argument of ‘show’, namely ‘(f 0)’ + In the expression: show (f 0) + In an equation for ‘x’: x = show (f 0) + ===================================== testsuite/tests/default/all.T ===================================== @@ -30,6 +30,7 @@ test('default-fail05', normal, compile_fail, ['']) test('default-fail06', normal, compile_fail, ['']) test('default-fail07', normal, compile_fail, ['']) test('default-fail08', normal, compile_fail, ['']) +test('T25775', normal, compile_fail, ['']) test('T25206', [extra_files(['T25206_helper.hs'])], multimod_compile, ['T25206', '']) test('T25858', normal, compile_and_run, ['']) test('T25858v1', [extra_files(['T25858v1_helper.hs'])], multimod_compile_and_run, ['T25858v1', '']) ===================================== testsuite/tests/typecheck/should_compile/T26451.hs ===================================== @@ -0,0 +1,34 @@ +{-# LANGUAGE ImplicitParams, TypeFamilies, FunctionalDependencies, ScopedTypeVariables #-} + +module T26451 where + +type family F a +type instance F Bool = [Char] + +class C a b | b -> a +instance C Bool Bool +instance C Char Char + +eq :: forall a b. C a b => a -> b -> () +eq p q = () + +g :: a -> F a +g = g + +f (x::tx) (y::ty) -- x :: alpha y :: beta + = let ?v = g x -- ?ip :: F alpha + in (?v::[ty], eq x True) + + +{- tx, and ty are unification variables + +Inert: [G] dg :: IP "v" (F tx) + [W] dw :: IP "v" [ty] +Work-list: [W] dc1 :: C tx Bool + [W] dc2 :: C ty Char + +* Solve dc1, we get tx := Bool from fundep +* Kick out dg +* Solve dg to get [G] dc : IP "v" [Char] +* Add that new dg to the inert set: that simply deletes dw!!! +-} ===================================== testsuite/tests/typecheck/should_compile/all.T ===================================== @@ -938,3 +938,4 @@ test('T23501b', normal, compile, ['']) test('T25597', normal, compile, ['']) test('T25960', normal, compile, ['']) test('T26256a', normal, compile, ['']) +test('T26451', normal, compile, ['']) ===================================== testsuite/tests/typecheck/should_fail/T12921.stderr ===================================== @@ -1,3 +1,34 @@ +T12921.hs:4:1: error: [GHC-39999] + • Ambiguous type variable ‘a0’ arising from an annotation + prevents the constraint ‘(GHC.Internal.Data.Data.Data + a0)’ from being solved. + Probable fix: use a type annotation to specify what ‘a0’ should be. + Potentially matching instances: + instance (GHC.Internal.Data.Data.Data a, + GHC.Internal.Data.Data.Data b) => + GHC.Internal.Data.Data.Data (Either a b) + -- Defined in ‘GHC.Internal.Data.Data’ + instance GHC.Internal.Data.Data.Data Ordering + -- Defined in ‘GHC.Internal.Data.Data’ + ...plus 17 others + ...plus 49 instances involving out-of-scope types + (use -fprint-potential-instances to see them all) + • In the annotation: + {-# ANN module "HLint: ignore Reduce duplication" #-} + +T12921.hs:4:16: error: [GHC-39999] + • Ambiguous type variable ‘a0’ arising from the literal ‘"HLint: ignore Reduce duplication"’ + prevents the constraint ‘(GHC.Internal.Data.String.IsString + a0)’ from being solved. + Probable fix: use a type annotation to specify what ‘a0’ should be. + Potentially matching instance: + instance (a ~ Char) => GHC.Internal.Data.String.IsString [a] + -- Defined in ‘GHC.Internal.Data.String’ + ...plus two instances involving out-of-scope types + (use -fprint-potential-instances to see them all) + • In the annotation: + {-# ANN module "HLint: ignore Reduce duplication" #-} T12921.hs:7:8: error: [GHC-88464] Variable not in scope: choice :: [a0] -> Int -> Int + View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/9930dbc4e69ce154c81f3f99209d55b... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/9930dbc4e69ce154c81f3f99209d55b... 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