July 2018 - ghc-tickets

[GHC] #15370: Typed hole panic on GHC 8.6 (tcTyVarDetails)
by GHC 06 Aug '18

06 Aug '18

#15370: Typed hole panic on GHC 8.6 (tcTyVarDetails) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 (Type checker) | Keywords: TypeInType, | Operating System: Unknown/Multiple TypedHoles | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following program panics on GHC 8.6 and HEAD: {{{#!hs {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeApplications #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} module Bug where import Data.Kind import Data.Type.Equality import Data.Void data family Sing :: forall k. k -> Type data (~>) :: Type -> Type -> Type infixr 0 ~> type family Apply (f :: k1 ~> k2) (x :: k1) :: k2 newtype instance Sing (f :: k1 ~> k2) = SLambda { applySing :: forall t. Sing t -> Sing (Apply f t) } mkRefl :: n :~: j mkRefl = Refl right :: forall (r :: (x :~: y) ~> z). Sing r -> () right no = case mkRefl @x @y of Refl -> applySing no _ }}} {{{ $ /opt/ghc/8.6.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.6.0.20180627 for x86_64-unknown-linux): tcTyVarDetails co_a1BG :: y_a1Bz[sk:1] ~# x_a1By[sk:1] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in ghc:Outputable pprPanic, called at compiler/basicTypes/Var.hs:497:22 in ghc:Var }}} On GHC 8.4, this simply errors: {{{ $ /opt/ghc/8.4.3/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:23:10: error: • Couldn't match type ‘n’ with ‘j’ ‘n’ is a rigid type variable bound by the type signature for: mkRefl :: forall k1 (n :: k1) (j :: k1). n :~: j at Bug.hs:22:1-17 ‘j’ is a rigid type variable bound by the type signature for: mkRefl :: forall k1 (n :: k1) (j :: k1). n :~: j at Bug.hs:22:1-17 Expected type: n :~: j Actual type: n :~: n • In the expression: Refl In an equation for ‘mkRefl’: mkRefl = Refl • Relevant bindings include mkRefl :: n :~: j (bound at Bug.hs:23:1) | 23 | mkRefl = Refl | ^^^^ Bug.hs:29:13: error: • Couldn't match type ‘Sing (Apply r t0)’ with ‘()’ Expected type: () Actual type: Sing (Apply r t0) • In the expression: applySing no _ In a case alternative: Refl -> applySing no _ In the expression: case mkRefl @x @y of { Refl -> applySing no _ } • Relevant bindings include no :: Sing r (bound at Bug.hs:27:7) right :: Sing r -> () (bound at Bug.hs:27:1) | 29 | Refl -> applySing no _ | ^^^^^^^^^^^^^^ Bug.hs:29:26: error: • Found hole: _ :: Sing t0 Where: ‘t0’ is an ambiguous type variable ‘y’, ‘x’, ‘k’ are rigid type variables bound by the type signature for: right :: forall k1 (x1 :: k1) (y1 :: k1) z (r :: (x1 :~: y1) ~> z). Sing r -> () at Bug.hs:(25,1)-(26,21) • In the second argument of ‘applySing’, namely ‘_’ In the expression: applySing no _ In a case alternative: Refl -> applySing no _ • Relevant bindings include no :: Sing r (bound at Bug.hs:27:7) right :: Sing r -> () (bound at Bug.hs:27:1) Valid substitutions include undefined :: forall (a :: TYPE r). GHC.Stack.Types.HasCallStack => a (imported from ‘Prelude’ at Bug.hs:7:8-10 (and originally defined in ‘GHC.Err’)) | 29 | Refl -> applySing no _ | ^ }}} Note that this is distinct from #15142, as this is still reproducible on HEAD, even after commit 030211d21207dabb7a4bf21cc9af6fa5eb066db1. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15370> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15439: Refactor `printMinimalImports` into two functions and reexport them
by GHC 06 Aug '18

06 Aug '18

#15439: Refactor `printMinimalImports` into two functions and reexport them -------------------------------------+------------------------------------- Reporter: chshersh | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: GHC API | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple imports,refactoring,source | plugins | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Currently the `printMinimalImports` function finds set of minimal imports and prints them into file. * https://github.com/ghc/ghc/blob/0f5a63e3d763f18c683f076e0e96396166863f56/co… But it's more convenient to have function that returns set of minimal imports that can later be used by external tools like source plugins without need to parse content of `.import` file. And without need to copy- paste existing function. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15439> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13368: Derive superclasses automatically if possible
by GHC 05 Aug '18

05 Aug '18

#13368: Derive superclasses automatically if possible -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #10607 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- So I don't forget, this is like #10607 along another axis. Instead of writing {{{#!hs newtype IO_ a = IO_ (IO a) deriving newtype (Functor, Applicative, Monad, MonadIO) }}} allow writing {{{#!hs newtype IO_ a = IO_ (IO a) deriving newtype MonadIO }}} with the same meaning, are there any technical restrictions to this? This gets annoying in large hierarchies where you need `Eq`, `Ord`, `Num`, `Fractional`, `Floating`, `Real` and `RealFrac` just to derive {{{#!hs newtype F32 = F32 Float deriving newtype RealFloat }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13368> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13555: Typechecker regression when combining PolyKinds and MonoLocalBinds
by GHC 03 Aug '18

03 Aug '18

#13555: Typechecker regression when combining PolyKinds and MonoLocalBinds -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.2.1 Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `lol-0.6.0.0` from Hackage currently fails to build with GHC 8.2.1 because of this regression. Here is a minimized example: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE InstanceSigs #-} {-# LANGUAGE MonoLocalBinds #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} module Crypto.Lol.Types.FiniteField (GF(..)) where import Data.Functor.Identity (Identity(..)) data T a type Polynomial a = T a newtype GF fp d = GF (Polynomial fp) type CRTInfo r = (Int -> r, r) type Tagged s b = TaggedT s Identity b newtype TaggedT s m b = TagT { untagT :: m b } class Reflects a i where value :: Tagged a i class CRTrans mon r where crtInfo :: Reflects m Int => TaggedT m mon (CRTInfo r) instance CRTrans Maybe (GF fp d) where crtInfo :: forall m . (Reflects m Int) => TaggedT m Maybe (CRTInfo (GF fp d)) crtInfo = undefined }}} This typechecks OK with GHC 8.0.2, but with 8.2.1, it complains: {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs GHCi, version 8.2.0.20170403: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Crypto.Lol.Types.FiniteField ( Bug.hs, interpreted ) Bug.hs:25:14: error: • Couldn't match type ‘k0’ with ‘k2’ because type variable ‘k2’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for: crtInfo :: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) at Bug.hs:25:14-79 Expected type: TaggedT m Maybe (CRTInfo (GF fp d)) Actual type: TaggedT m Maybe (CRTInfo (GF fp d)) • When checking that instance signature for ‘crtInfo’ is more general than its signature in the class Instance sig: forall (m :: k0). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) Class sig: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) In the instance declaration for ‘CRTrans Maybe (GF fp d)’ | 25 | crtInfo :: forall m . (Reflects m Int) => TaggedT m Maybe (CRTInfo (GF fp d)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:25:14: error: • Could not deduce (Reflects m Int) from the context: Reflects m Int bound by the type signature for: crtInfo :: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) at Bug.hs:25:14-79 The type variable ‘k0’ is ambiguous • When checking that instance signature for ‘crtInfo’ is more general than its signature in the class Instance sig: forall (m :: k0). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) Class sig: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) In the instance declaration for ‘CRTrans Maybe (GF fp d)’ | 25 | crtInfo :: forall m . (Reflects m Int) => TaggedT m Maybe (CRTInfo (GF fp d)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} Notably, both `PolyKinds` and `MonoLocalBinds` are required to trigger this bug. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13555> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15141: decideKindGeneralisationPlan is too complicated
by GHC 02 Aug '18

02 Aug '18

#15141: decideKindGeneralisationPlan is too complicated -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider this program, which originally comes from #14846. I have decorated it with kind signatures so you can see what is going on {{{ {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE EmptyCase #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE InstanceSigs #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeInType #-} module T14846 where import Data.Kind import Data.Proxy -- Cat :: * -> * type Cat ob = ob -> ob -> Type -- Struct :: forall k. (k -> Constraint) -> * data Struct :: (k -> Constraint) -> Type where S :: Proxy (a::k) -> Struct (cls::k -> Constraint) -- Structured :: forall k -- forall (a:k) (cls:k->Constraint) -> Struct k cls type Structured a cls = (S ('Proxy :: Proxy a) :: Struct cls) -- AStruct :: forall k (cls : k->Constraint). Struct cls -> Type data AStruct :: Struct cls -> Type where AStruct :: cls a => AStruct (Structured a cls) -- StructI :: forall k1 k2 (cls :: k2 -> Constraint). -- k1 -> Struct k2 cls -> Constraint class StructI xx (structured::Struct (cls :: k -> Constraint)) where struct :: AStruct structured -- Hom :: forall k1 k2 (cls :: k2 -> Constraint). -- Cat k1 -> Cat (Struct {k2} cls) data Hom :: Cat k -> Cat (Struct cls) where -- Category :: forall ob. Cat ob -> Constraint class Category (cat::Cat ob) where i :: StructI xx a => ríki a a instance forall (riki :: Cat kx) (cls :: kk -> Constraint). Category riki => Category (Hom riki :: Cat (Struct cls)) where i :: forall xx a. StructI xx a => Hom riki a a i = case struct :: AStruct (Structured a cls) of }}} Focus on the final instance declaration. Do kind inference on the type signature for `i`. I think we should get {{{ i :: forall {k1} {k2} {cls1 :: k2 -> Constraint} forall (xx :: k1) (a :: Struct {k2} cls1). StructI {k1} {k2} xx a => Hom {kx} {k2} riki a a }}} Nothing about `i`'s type signature constraints the `cls` to be the same as the in-scope `cls`. Yes `riki` is in scope, but its kind is unrelated. But at present GHC does not kind-generalise `i`'s type (because of `kindGeneralisationPlan`), so random things in the body of `i` may influence how its kinds get fixed. And if we changed the signature a bit so that it didn't mention `riki` any more, suddenly it ''would'' be kind-generalised. This seems incomprehensibly complicated to me. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15141> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10087: DefaultSignatures: error message mentions internal name
by GHC 02 Aug '18

02 Aug '18

#10087: DefaultSignatures: error message mentions internal name -------------------------------------+------------------------------------- Reporter: | Owner: andreas.abel | Status: new Type: feature | Milestone: request | Version: 7.8.4 Priority: normal | Operating System: Unknown/Multiple Component: Compiler | Type of failure: Other (Type checker) | Blocked By: Keywords: | Related Tickets: Architecture: | Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE DefaultSignatures #-} class C a where reflexive :: a -> Bool default reflexive :: Eq a => a -> Bool reflexive x = x == x data D instance C D where -- /home/abel/play/haskell/bugs/DefaultSig.hs:10:10: -- No instance for (Eq D) arising from a use of ‘Main.$gdmreflexive’ -- In the expression: Main.$gdmreflexive -- In an equation for ‘reflexive’: reflexive = Main.$gdmreflexive -- In the instance declaration for ‘C D’ }}} Error looks odd: The user has not written $gdmreflexive in his code. TODO: Better error message. Maybe this should just trigger a warning that method {{{reflexive}}} is undefined for instance {{{D}}} of {{{C}}}. Like when I remove the default method. {{{ /home/abel/play/haskell/bugs/DefaultSig.hs:10:10: Warning: No explicit implementation for ‘reflexive’ In the instance declaration for ‘C D’ }}} It seems the semantics of a default signature is that each instance *must* implement this method. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10087> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12096: Attach stacktrace information to SomeException
by GHC 02 Aug '18

02 Aug '18

#12096: Attach stacktrace information to SomeException -------------------------------------+------------------------------------- Reporter: ndtimofeev | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: | Version: 8.0.1 libraries/base | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Now (base 4.9.0.0) ErrorCall contain field for stacktrace information and error fills it. If you add field for stacktrace information to SomeException {{{#!hs data SomeException where SomeExceptionWithLocation :: Exception e => String -> e -> SomeException pattern SomeException :: () => Exception e => e -> SomeException pattern SomeException err <- SomeExceptionWithLocation _ err where SomeException err = SomeExceptionWithLocation "" err }}} and will fills it in throw {{{#!hs throw :: (HasCallStack, Exception e) => e -> a throw e = unsafeDupablePerformIO $ do stack <- currentCallStack raise# (CallStackException e $ if stack /= [] then prettyCallStack ?callStack ++ "\n" ++ renderStack stack else prettyCallStack ?callStack) }}} it will be more useful for ghci users. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12096> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11759: can't decompose ghc.exe path
by GHC 02 Aug '18

02 Aug '18

#11759: can't decompose ghc.exe path --------------------------------------+---------------------------- Reporter: erisco | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Windows Architecture: x86_64 (amd64) | Type of failure: Other Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+---------------------------- {{{ erisco@ERICWIN C:\Users\erisco\haskell-platform-bin $ ghc.exe ghc.exe: panic! (the 'impossible' happened) (GHC version 7.8.3 for x86_64-unknown-mingw32): can't decompose ghc.exe path: "C:\\Users\\erisco\\haskell- platform-bin\\ghc.exe" }}} haskell-platform-bin is a symbolic directory link to "C:\Program Files\Haskell Platform\2014.2.0.0\bin". I did this so there would not be spaces in the path because this is causing other issues for me. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11759> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15385: -Wincomplete-patterns gets confused when combining GADTs and pattern guards
by GHC 01 Aug '18

01 Aug '18

#15385: -Wincomplete-patterns gets confused when combining GADTs and pattern guards -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple PatternMatchWarnings | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following code: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE UndecidableInstances #-} {-# OPTIONS_GHC -Wincomplete-patterns #-} module Bug where import Data.Kind data family Sing :: forall k. k -> Type newtype Id a = MkId a data So :: Bool -> Type where Oh :: So True data instance Sing :: Bool -> Type where SFalse :: Sing False STrue :: Sing True data instance Sing :: Ordering -> Type where SLT :: Sing LT SEQ :: Sing EQ SGT :: Sing GT data instance Sing :: forall a. Id a -> Type where SMkId :: Sing x -> Sing (MkId x) class POrd a where type (x :: a) `Compare` (y :: a) :: Ordering class SOrd a where sCompare :: forall (x :: a) (y :: a). Sing x -> Sing y -> Sing (x `Compare` y) type family (x :: a) <= (y :: a) :: Bool where x <= y = LeqCase (x `Compare` y) type family LeqCase (o :: Ordering) :: Bool where LeqCase LT = True LeqCase EQ = True LeqCase GT = False (%<=) :: forall a (x :: a) (y :: a). SOrd a => Sing x -> Sing y -> Sing (x <= y) sx %<= sy = case sx `sCompare` sy of SLT -> STrue SEQ -> STrue SGT -> SFalse class (POrd a, SOrd a) => VOrd a where leqReflexive :: forall (x :: a). Sing x -> So (x <= x) instance POrd a => POrd (Id a) where type MkId x `Compare` MkId y = x `Compare` y instance SOrd a => SOrd (Id a) where SMkId sx `sCompare` SMkId sy = sx `sCompare` sy ----- instance VOrd a => VOrd (Id a) where leqReflexive (SMkId sa) | Oh <- leqReflexive sa = case sa `sCompare` sa of SLT -> Oh SEQ -> Oh -- SGT -> undefined }}} What exactly this code does isn't terribly important. The important bit is that last instance (`VOrd (Id a)`). That //should// be total, but GHC disagrees: {{{ GHCi, version 8.6.0.20180627: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:63:7: warning: [-Wincomplete-patterns] Pattern match(es) are non-exhaustive In a case alternative: Patterns not matched: SGT | 63 | = case sa `sCompare` sa of | ^^^^^^^^^^^^^^^^^^^^^^^^... }}} As evidence that this warning is bogus, if you uncomment the last line, then GHC correctly observes that that line is inaccessible: {{{ GHCi, version 8.6.0.20180627: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:66:9: warning: [-Winaccessible-code] • Couldn't match type ‘'True’ with ‘'False’ Inaccessible code in a pattern with constructor: SGT :: Sing 'GT, in a case alternative • In the pattern: SGT In a case alternative: SGT -> undefined In the expression: case sa `sCompare` sa of SLT -> Oh SEQ -> Oh SGT -> undefined | 66 | SGT -> undefined | ^^^ }}} Clearly, something is afoot in the coverage checker. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15385> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15380: Infinite typechecker loop in GHC 8.6
by GHC 01 Aug '18

01 Aug '18

#15380: Infinite typechecker loop in GHC 8.6 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 (Type checker) | Keywords: TypeInType, | Operating System: Unknown/Multiple TypeFamilies | Architecture: | Type of failure: Compile-time Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following program loops infinitely during typechecking with GHC 8.6.1 and HEAD: {{{#!hs {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} module Bug where import Data.Kind class Generic a where type Rep a :: Type class PGeneric a where type To a (x :: Rep a) :: a type family MDefault (x :: a) :: a where MDefault x = To (M x) class C a where type M (x :: a) :: a type M (x :: a) = MDefault x }}} In GHC 8.4.3, however this fails with a proper error: {{{ $ /opt/ghc/8.4.3/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:15:16: error: • Occurs check: cannot construct the infinite kind: a ~ Rep (M x) -> M x • In the type ‘To (M x)’ In the type family declaration for ‘MDefault’ • Type variable kinds: x :: a | 15 | MDefault x = To (M x) | ^^^^^^^^ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15380> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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