June 2017 - ghc-tickets

[GHC] #13887: Template Haskell pretty-printer doesn't parenthesize infix datatype names in data declarations
by GHC 29 Jul '17

29 Jul '17

#13887: Template Haskell pretty-printer doesn't parenthesize infix datatype names in data declarations -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Template | Version: 8.0.1 Haskell | Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If you run this program: {{{#!hs {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} module Foo where import Language.Haskell.TH main :: IO () main = do putStrLn $([d| data a :~: b where Refl1 :: a :~: a |] >>= stringE . pprint) putStrLn $([d| data a :~~: b = a ~ b => Refl2 |] >>= stringE . pprint) }}} {{{ $ /opt/ghc/8.2.1/bin/runghc Foo.hs data :~:_0 a_1 b_2 where Refl1_3 :: :~:_0 a_4 a_4 data :~~:_0 a_1 b_2 = a_1 ~ b_2 => Refl2_3 }}} It'll print the output incorrectly. Those infix names `:~:` and `:~~:` ought to be surrounded by parentheses, since they're used in prefix position. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13887> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13173: Investigate old comment at the top of SrcLoc
by GHC 29 Jul '17

29 Jul '17

#13173: Investigate old comment at the top of SrcLoc -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: task | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 8.0.1 Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- There is a comment at the top of `SrcLoc` which says {{{ {-# OPTIONS_GHC -fno-omit-interface-pragmas #-} -- Workaround for Trac #5252 crashes the bootstrap compiler without -O -- When the earliest compiler we want to boostrap with is -- GHC 7.2, we can make RealSrcLoc properly abstract }}} This is a well-defined small task so I will mark it for newcomers. 1. Try to work out what the pragma is meant to do by reading #5252 2. Work out whether we can now make `RealSrcLoc` "properly abstract" (if it is not already) 3. Remove or update this comment and pragma. 4. Pray it doesn't break the whole compiler. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13173> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13710: panic with boot and -jX
by GHC 29 Jul '17

29 Jul '17

#13710: panic with boot and -jX -------------------------------------+------------------------------------- Reporter: pacak | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{ % cat A.hs module A where }}} {{{ % cat A.hs-boot module A ( A ) where type A = Maybe () yay :: A -> IO () }}} {{{ % cat B.hs module B where import {-# SOURCE #-} A }}} This panics: {{{ % rm -rf o ; ghc -j2 -outputdir o B [1 of 3] Compiling A[boot] ( A.hs-boot, o/A.o-boot ) [2 of 3] Compiling A ( A.hs, o/A.o ) <no location info>: error: ghc: panic! (the 'impossible' happened) (GHC version 8.2.0.20170503 for x86_64-unknown-linux): tcIfaceGlobal (local): not found You are in a maze of twisty little passages, all alike. While forcing the thunk for TyThing A which was lazily initialized by initIfaceCheck typecheckLoop, I tried to tie the knot, but I couldn't find A in the current type environment. If you are developing GHC, please read Note [Tying the knot] and Note [Type-checking inside the knot]. Consider rebuilding GHC with profiling for a better stack trace. Contents of current type environment: [r15J :-> Identifier ‘$trModule’] Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1134:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1138:37 in ghc:Outputable pprPanic, called at compiler/iface/TcIface.hs:1689:23 in ghc:TcIface Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} This works: {{{ % rm -rf o ; ghc -j1 -outputdir o B [1 of 3] Compiling A[boot] ( A.hs-boot, o/A.o-boot ) [2 of 3] Compiling A ( A.hs, o/A.o ) A.hs-boot:3:1: error: ‘A.A’ is exported by the hs-boot file, but not exported by the module | 3 | type A = Maybe () | ^^^^^^^^^^^^^^^^^ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13710> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12989: ($) can have a more general type
by GHC 28 Jul '17

28 Jul '17

#12989: ($) can have a more general type -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The documentation (in section 9.12.1 for GHC 8; I don't know what section it is in 8.2) suggests a hypothetical type for `($)`: {{{#!hs bad :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep) (a :: TYPE r1) (b :: TYPE r2). (a -> b) -> a -> b bad f x = f x }}} It explains, correctly, that this definition will not work because `x` has a representation-polymorphic type. However, this doesn't actually explain why `($)` doesn't have that type! Indeed, both GHC 8 and 8.2 accept the following: {{{#!hs good :: forall (r1 :: RuntimeRep) (r2 :: RuntimeRep) (a :: TYPE r1) (b :: TYPE r2). (a -> b) -> a -> b good f = f }}} This has very slightly different semantics, but anyone relying on the difference is already doing something they shouldn't. It may or may not be possible to retain the current semantics with some care and magic, if we so desire. I didn't really know how to characterize this ticket properly. It could be considered a documentation bug or a compiler feature request. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12989> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13541: Make it easier to use the gold linker
by GHC 28 Jul '17

28 Jul '17

#13541: Make it easier to use the gold linker -------------------------------------+------------------------------------- Reporter: bgamari | 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: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- As pointed out in #4862, the `gold` linker is significantly faster than BFD `ld`. Currently we use whatever linker `gcc` uses by default. This is an unfortunate situation for users as few packagers take the effort to configure their builds to use `gold`. I think we should consider the following, > Introduce a configure flag (to both the source distribution, and the distributed binary distributions), `--enable-gold`. When enabled, `configure` will check for the functioning of `gcc -fuse-ld=gold`. If found to work `-fuse-ld=gold` would be added to GHC's `optl`. The flag would throw an error on non-ELF platforms (which are not supported by `gold`). While there is admittedly not a whole lot of precedent for this, the status quo means we are leaving a significant bit of compiler performance on the table in a majority of cases. Given that `stack` uses GHC's official bindists, we should try to improve this situation. * -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13541> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13837: Calling qReifyInstances on out-of-scope Name leads to GHC internal error
by GHC 28 Jul '17

28 Jul '17

#13837: Calling qReifyInstances on out-of-scope Name leads to GHC internal error -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Template | Version: 8.0.1 Haskell | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE TemplateHaskell #-} module Bug where import Language.Haskell.TH.Lib import Language.Haskell.TH.Syntax test_local_tyfam_expansion :: String test_local_tyfam_expansion = $(do fam_name <- newName "Fam" stringE . show =<< qReifyInstances fam_name []) }}} {{{ $ /opt/ghc/8.2.1/bin/ghci Bug2.hs GHCi, version 8.2.0.20170523: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug2.hs, interpreted ) Bug2.hs:9:5: error: • The exact Name ‘Fam_a4pX’ is not in scope Probable cause: you used a unique Template Haskell name (NameU), perhaps via newName, but did not bind it If that's it, then -ddump-splices might be useful • In the argument of reifyInstances: Fam_0 In the untyped splice: $(do fam_name <- newName "Fam" stringE . show =<< qReifyInstances fam_name []) | 9 | $(do fam_name <- newName "Fam" | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Bug2.hs:9:5: error: • GHC internal error: ‘Fam_a4pX’ is not in scope during type checking, but it passed the renamer tcl_env of environment: [] • In the type ‘Fam_a4pX’ In the argument of reifyInstances: Fam_0 In the untyped splice: $(do fam_name <- newName "Fam" stringE . show =<< qReifyInstances fam_name []) | 9 | $(do fam_name <- newName "Fam" | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... }}} This appears to have started happening in GHC 7.10, since with 7.8, you only get this: {{{ GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Bug ( Bug2.hs, interpreted ) Loading package array-0.5.0.0 ... linking ... done. Loading package deepseq-1.3.0.2 ... linking ... done. Loading package containers-0.5.5.1 ... linking ... done. Loading package pretty-1.1.1.1 ... linking ... done. Loading package template-haskell ... linking ... done. Bug2.hs:9:5: The exact Name ‘Fam_a2gK’ is not in scope Probable cause: you used a unique Template Haskell name (NameU), perhaps via newName, but did not bind it If that's it, then -ddump-splices might be useful In the argument of reifyInstances: Fam_0 In the splice: $(do { fam_name <- newName "Fam"; stringE . show =<< qReifyInstances fam_name [] }) }}} This problem appears to be somewhat specific to `qReifyInstances`, since switching it out with `qReify` does not trigger the internal error: {{{#!hs {-# LANGUAGE TemplateHaskell #-} module Bug where import Language.Haskell.TH.Lib import Language.Haskell.TH.Syntax test_local_tyfam_expansion :: String test_local_tyfam_expansion = $(do fam_name <- newName "Fam" stringE . show =<< qReify fam_name) }}} {{{ GHCi, version 8.2.0.20170523: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug2.hs, interpreted ) Bug2.hs:9:5: error: • The exact Name ‘Fam_a4od’ is not in scope Probable cause: you used a unique Template Haskell name (NameU), perhaps via newName, but did not bind it If that's it, then -ddump-splices might be useful • In the untyped splice: $(do fam_name <- newName "Fam" stringE . show =<< qReify fam_name) | 9 | $(do fam_name <- newName "Fam" | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Bug2.hs:9:5: error: • The exact Name ‘Fam_a4od’ is not in scope Probable cause: you used a unique Template Haskell name (NameU), perhaps via newName, but did not bind it If that's it, then -ddump-splices might be useful • In the untyped splice: $(do fam_name <- newName "Fam" stringE . show =<< qReify fam_name) | 9 | $(do fam_name <- newName "Fam" | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Bug2.hs:9:5: error: • ‘Fam_a4od’ is not in the type environment at a reify • In the untyped splice: $(do fam_name <- newName "Fam" stringE . show =<< qReify fam_name) | 9 | $(do fam_name <- newName "Fam" | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13837> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13601: GHC errors but hangs
by GHC 28 Jul '17

28 Jul '17

#13601: GHC errors but hangs -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new 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: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE TypeFamilies, DataKinds, TypeInType #-} import GHC.Exts import Prelude (Bool(True,False),Integer,Ordering,undefined) import qualified Prelude import Data.Kind -------------------- -- class hierarchy type family Rep (rep :: RuntimeRep) :: RuntimeRep where -- Rep IntRep = IntRep -- Rep DoubleRep = IntRep -- Rep PtrRepUnlifted = IntRep -- Rep PtrRepLifted = PtrRepLifted class Boolean (Logic a) => Eq (a :: TYPE rep) where type Logic (a :: TYPE rep) :: TYPE (Rep rep) (==) :: a -> a -> Logic a class Eq a => POrd (a :: TYPE rep) where inf :: a -> a -> a class POrd a => MinBound (a :: TYPE rep) where minBound :: () -> a class POrd a => Lattice (a :: TYPE rep) where sup :: a -> a -> a class (Lattice a, MinBound a) => Bounded (a :: TYPE rep) where maxBound :: () -> a class Bounded a => Complemented (a :: TYPE rep) where not :: a -> a class Bounded a => Heyting (a :: TYPE rep) where infixr 3 ==> (==>) :: a -> a -> a class (Complemented a, Heyting a) => Boolean a (||) :: Boolean a => a -> a -> a (||) = sup (&&) :: Boolean a => a -> a -> a (&&) = inf }}} hangs with {{{ $ ghci a.hs GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( a.hs, interpreted ) a.hs:18:16: error: C-c C-cInterrupted. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13601> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13780: Nightmarish pretty-printing of equality type in GHC 8.2 error message
by GHC 28 Jul '17

28 Jul '17

#13780: Nightmarish pretty-printing of equality type in GHC 8.2 error message -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 (Type checker) | Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I originally spotted this in https://ghc.haskell.org/trac/ghc/ticket/12102#comment:1, but I happened to stumble upon it again recently in a separate context, so I though it deserved its own ticket. Here's some code which does not typecheck: {{{#!hs {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} module Foo where data family Sing (a :: k) data Foo a = a ~ Bool => MkFoo data instance Sing (z :: Foo a) = (z ~ MkFoo) => SMkFoo }}} In GHC 8.0.1 and 8.0.2, the error message you'd get from this program was reasonable enough: {{{ GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Foo ( Bug.hs, interpreted ) Bug.hs:9:40: error: • Expected kind ‘Foo a’, but ‘'MkFoo’ has kind ‘Foo Bool’ • In the second argument of ‘~’, namely ‘MkFoo’ In the definition of data constructor ‘SMkFoo’ In the data instance declaration for ‘Sing’ }}} But in GHC 8.2.1 and HEAD, it's hair-raisingly bad: {{{ GHCi, version 8.2.0.20170522: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Foo ( Bug.hs, interpreted ) Bug.hs:9:40: error: • Expected kind ‘Foo a’, but ‘'MkFoo ('Data.Type.Equality.C:~ ('GHC.Types.Eq# <>))’ has kind ‘Foo Bool’ • In the second argument of ‘~’, namely ‘MkFoo’ In the definition of data constructor ‘SMkFoo’ In the data instance declaration for ‘Sing’ | 9 | data instance Sing (z :: Foo a) = (z ~ MkFoo) => SMkFoo | ^^^^^ }}} **WAT.** -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13780> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13877: GHC panic: No skolem info: k2
by GHC 28 Jul '17

28 Jul '17

#13877: GHC panic: No skolem info: k2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple 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 code causes a GHC panic on GHC 8.0.1, 8.0.2, 8.2.1, and HEAD: {{{#!hs {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE Trustworthy #-} {-# LANGUAGE TypeApplications #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} module Eliminator where import Data.Kind data family Sing (a :: k) data instance Sing (z :: [a]) where SNil :: Sing '[] SCons :: Sing x -> Sing xs -> Sing (x:xs) data TyFun :: * -> * -> * type a ~> b = TyFun a b -> * infixr 0 ~> type family Apply (f :: k1 ~> k2) (x :: k1) :: k2 type a @@ b = Apply a b infixl 9 @@ data FunArrow = (:->) | (:~>) class FunType (arr :: FunArrow) where type Fun (k1 :: Type) arr (k2 :: Type) :: Type class FunType arr => AppType (arr :: FunArrow) where type App k1 arr k2 (f :: Fun k1 arr k2) (x :: k1) :: k2 type FunApp arr = (FunType arr, AppType arr) instance FunType (:->) where type Fun k1 (:->) k2 = k1 -> k2 $(return []) -- This is only necessary for GHC 8.0 -- GHC 8.2 is smarter instance AppType (:->) where type App k1 (:->) k2 (f :: k1 -> k2) x = f x instance FunType (:~>) where type Fun k1 (:~>) k2 = k1 ~> k2 $(return []) instance AppType (:~>) where type App k1 (:~>) k2 (f :: k1 ~> k2) x = f @@ x infixr 0 -?> type (-?>) (k1 :: Type) (k2 :: Type) (arr :: FunArrow) = Fun k1 arr k2 listElim :: forall (a :: Type) (p :: [a] -> Type) (l :: [a]). Sing l -> p '[] -> (forall (x :: a) (xs :: [a]). Sing x -> Sing xs -> p xs -> p (x:xs)) -> p l listElim = listElimPoly @(:->) @a @p @l listElimTyFun :: forall (a :: Type) (p :: [a] ~> Type) (l :: [a]). Sing l -> p @@ '[] -> (forall (x :: a) (xs :: [a]). Sing x -> Sing xs -> p @@ xs -> p @@ (x:xs)) -> p @@ l -- The line below causes a GHC panic. It should not typecheck; -- uncomment the line below it for the correct version listElimTyFun = listElimPoly @(:->) @a @p @l -- listElimTyFun = listElimPoly @(:~>) @a @p @l listElimPoly :: forall (arr :: FunArrow) (a :: Type) (p :: ([a] -?> Type) arr) (l :: [a]). FunApp arr => Sing l -> App [a] arr Type p '[] -> (forall (x :: a) (xs :: [a]). Sing x -> Sing xs -> App [a] arr Type p xs -> App [a] arr Type p (x:xs)) -> App [a] arr Type p l listElimPoly SNil pNil _ = pNil listElimPoly (SCons x (xs :: Sing xs)) pNil pCons = pCons x xs (listElimPoly @arr @a @p @xs xs pNil pCons) }}} {{{ $ /opt/ghc/8.2.1/bin/ghci Eliminator.hs GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Eliminator ( Eliminator.hs, interpreted ) Eliminator.hs:72:17: error:ghc: panic! (the 'impossible' happened) (GHC version 8.2.0.20170623 for x86_64-unknown-linux): No skolem info: k2_a5cr Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcErrors.hs:2653:5 in ghc:TcErrors Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13877> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13768: Incorrect warnings generated by exhaustiveness checker with pattern synonyms / GADT combination
by GHC 28 Jul '17

28 Jul '17

#13768: Incorrect warnings generated by exhaustiveness checker with pattern synonyms / GADT combination -------------------------------------+------------------------------------- Reporter: kosmikus | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 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: -------------------------------------+------------------------------------- The module {{{ {-# LANGUAGE DataKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE ViewPatterns #-} module PatSynEx where data NS (f :: k -> *) (xs :: [k]) = NS Int data IsNS (f :: k -> *) (xs :: [k]) where IsZ :: f x -> IsNS f (x ': xs) IsS :: NS f xs -> IsNS f (x ': xs) isNS :: NS f xs -> IsNS f xs isNS = undefined pattern Z :: () => (xs' ~ (x ': xs)) => f x -> NS f xs' pattern Z x <- (isNS -> IsZ x) pattern S :: () => (xs' ~ (x ': xs)) => NS f xs -> NS f xs' pattern S p <- (isNS -> IsS p) {-# COMPLETE Z, S #-} data SList :: [k] -> * where SNil :: SList '[] SCons :: SList (x ': xs) go :: SList ys -> NS f ys -> Int go SCons (Z _) = 0 go SCons (S _) = 1 go SNil _ = error "inaccessible" }}} produces the following warnings with both GHC 8.2 and 8.3: {{{ PatSynEx.hs:31:1: warning: [-Woverlapping-patterns] Pattern match has inaccessible right hand side In an equation for ‘go’: go SCons (Z _) = ... | 31 | go SCons (Z _) = 0 | ^^^^^^^^^^^^^^^^^^ PatSynEx.hs:32:1: warning: [-Woverlapping-patterns] Pattern match is redundant In an equation for ‘go’: go SCons (S _) = ... | 32 | go SCons (S _) = 1 | ^^^^^^^^^^^^^^^^^^ }}} Neither warning seems correct. The first case is not inaccessible, the second case is not redundant. The catch-all case has always been required with GHC even though it is not really reachable, but that's not the subject of this issue. I tried to simplify the issue further by getting rid of the `f` argument in `NS` and `IsNS`, but that made the issue disappear. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13768> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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