September 2018 - ghc-tickets

[GHC] #15526: Explain or remove mystery import in Unsafe.Coerce
by GHC 05 Sep '18

05 Sep '18

#15526: Explain or remove mystery import in Unsafe.Coerce -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.8.1 Component: Core | Version: 8.4.3 Libraries | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `Unsafe.Coerce` has the line {{{#!hs import GHC.Integer () -- for build ordering }}} It is not at all clear to me why build ordering demands that `GHC.Integer` be compiled before `Unsafe.Coerce` when the latter uses no numeric literals, no arithmetic, and no numeric types. If there's a real reason for this, the comment should be expanded. Otherwise, the import should be removed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15526> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into a hole' panic
by GHC 05 Sep '18

05 Sep '18

#15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into a hole' panic -------------------------------------+------------------------------------- Reporter: dramforever | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 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: -------------------------------------+------------------------------------- This program, when compiled like `ghc -fdefer-type-errors -c panic.hs`: {{{#!hs {-# LANGUAGE TypeFamilies #-} module MorePanic where transitive :: (a, b) -> (b, c) -> (a, c) transitive = undefined trigger :: a -> b -> (F a b, F b a) trigger _ _ = _ `transitive` trigger _ _ type family F (n :: *) (m :: *) :: * }}} Gives this panic after several error messages: {{{ ghc.EXE: panic! (the 'impossible' happened) (GHC version 8.4.3 for x86_64-unknown-mingw32): opt_univ fell into a hole {co_a7Sj} Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler\utils\Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler\\types\\OptCoercion.hs:242:5 in ghc:OptCoercion Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} This bug also affects GHCi. Attached is full output -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15368> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10249: Suboptimal error message with deferred type errors
by GHC 05 Sep '18

05 Sep '18

#10249: Suboptimal error message with deferred type errors -------------------------------------+------------------------------------- Reporter: | Owner: Iceland_jack | Status: new Type: feature | Milestone: request | Version: 7.10.1 Priority: normal | Operating System: Unknown/Multiple Component: GHCi | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- {{{#!hs $ ghci -fdefer-type-errors -ignore-dot-ghci GHCi, version 7.10.0.20150316: http://www.haskell.org/ghc/ :? for help Prelude> _ <interactive>:2:1: Warning: Found hole ‘_’ with type: IO t0 Where: ‘t0’ is an ambiguous type variable In the first argument of ‘GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly’, namely ‘_’ In a stmt of an interactive GHCi command: it <- GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly _ *** Exception: <interactive>:2:1: Found hole ‘_’ with type: IO t0 Where: ‘t0’ is an ambiguous type variable In the first argument of ‘GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly’, namely ‘_’ In a stmt of an interactive GHCi command: it <- GHC.GHCi.ghciStepIO :: IO a_aly -> IO a_aly _ (deferred type error) Prelude> }}} It should ideally not expose `ghciStepIO` to the user. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10249> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15605: Documentation of atomicModifyMutVar# does not show properly
by GHC 05 Sep '18

05 Sep '18

#15605: Documentation of atomicModifyMutVar# does not show properly -------------------------------------+------------------------------------- Reporter: harendra | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: libraries | Version: 8.4.3 (other) | Keywords: ghc-prim | 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 actual documentation in the source is: {{{ -- | Modify the contents of a @MutVar\#@. Note that this isn\'t strictly -- speaking the correct type for this function, it should really be -- @MutVar# s a -> (a -> (a,b)) -> State# s -> (# State# s, b #)@, however -- we don\'t know about pairs here. }}} It actually shows as this in the docs, note the incomplete signature: {{{ Modify the contents of a MutVar#. Note that this isn't strictly speaking the correct type for this function, it should really be MutVar s -> ( s, b #), however we don't know about pairs here. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15605> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4017: Unhelpful error message in GHCi
by GHC 05 Sep '18

05 Sep '18

#4017: Unhelpful error message in GHCi -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 6.12.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: #13862 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RolandSenn): * related: => #13862 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4017#comment:17> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #3427: control what sort of entity a deprecated pragma applies to
by GHC 05 Sep '18

05 Sep '18

#3427: control what sort of entity a deprecated pragma applies to -------------------------------------+------------------------------------- Reporter: igloo | Owner: nineonine Type: feature request | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 6.10.4 Resolution: | Keywords: deprecate | warning, newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by nineonine): * status: new => patch -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3427#comment:20> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10756: Allow users to indicate inaccessible patterns
by GHC 04 Sep '18

04 Sep '18

#10756: Allow users to indicate inaccessible patterns -------------------------------------+------------------------------------- Reporter: edsko | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider {{{#!hs {-# LANGUAGE KindSignatures #-} {-# LANGUAGE GADTs #-} data Elem :: * -> * -> * where EZ :: Elem (f , fs) f ES :: Elem fs f -> Elem (f', fs) f elemAbsurd :: Elem () f -> a elemAbsurd _ = error "inaccessible" }}} This catch-all in `elemAbsurd` is very unfortunate, because if I change my datatype and not all cases are inaccessible anymore the type checker will not be warning me. Although of course there is recent work by SPJ et al on inferring more inaccessible cases, I think it would be tremendously useful for users to be able to indicate that patterns are in fact inaccesible. I'd be very happy if I could write {{{#!hs elemAbsurd :: Elem () f -> a elemAbsurd EZ = inaccessible elemAbsurd (ES _) = inaccessible }}} Note that ghc ''can already detect that these patterns are in fact inaccessible'': it won't let me write the above: {{{ T.hs:12:12: Couldn't match type ‘()’ with ‘(f, fs)’ Inaccessible code in a pattern with constructor EZ :: forall f fs. Elem (f, fs) f, in an equation for ‘elemAbsurd’ }}} So I think it should be very very simple to change the type checker to allow to write cases like this if and only if the RHS is a specially designated symbol "inaccessible", which could be defined simply as {{{#!hs inaccessible :: a inaccessible = error "inaccessible" }}} in `GHC.Prim` or something. This would seem like very little work, and a correspondingly tiny patch, but with quite a large payoff. Future work in automatically detecting inaccessible cases would not make existing code using this style wrong, it just means that we have to write fewer inaccessible cases by hand; but even with more sophisticated inference algorithms for inaccessible cases there will still be inaccessible cases remaining. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10756> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #3483: Some mechanism for eliminating "absurd" patterns
by GHC 04 Sep '18

04 Sep '18

#3483: Some mechanism for eliminating "absurd" patterns -------------------------------------+------------------------------------- Reporter: ryani | Owner: (none) Type: feature request | Status: new Priority: low | Milestone: Component: Compiler | Version: 6.10.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10756 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by monoidal): * related: => #10756 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3483#comment:12> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15570: Core transformations generate bad indexCharOffAddr# call
by GHC 04 Sep '18

04 Sep '18

#15570: Core transformations generate bad indexCharOffAddr# call -------------------------------------+------------------------------------- Reporter: alpmestan | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 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 the following functions, which only differ in a bang pattern on the local binding `q` in the inner loop: {{{#!hs {-# LANGUAGE BangPatterns #-} {-# LANGUAGE MagicHash #-} module Bug where import GHC.Prim import GHC.Types f :: Int -> String f n_ = go n_ "" where go n cs | n < 62 = let !c = chooseChar62 n in c : cs | otherwise = go q (c : cs) where (q, r) = quotRem n 62 !c = chooseChar62 r chooseChar62 :: Int -> Char {-# INLINE chooseChar62 #-} chooseChar62 (I# n) = C# (indexCharOffAddr# chars62 n) chars62 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"# g :: Int -> String g n_ = go n_ "" where go n cs | n < 62 = let !c = chooseChar62 n in c : cs | otherwise = go q (c : cs) where (!q, r) = quotRem n 62 -- !!! Note the bang on q !c = chooseChar62 r chooseChar62 :: Int -> Char {-# INLINE chooseChar62 #-} chooseChar62 (I# n) = C# (indexCharOffAddr# chars62 n) chars62 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"# }}} When building with `-O -fPIC -dynamic -ddump-simpl`, this is the Core I see, with a HEAD checkout from earlier this week built by hadrian: {{{#!hs -- chararacter array, used by both chars62_r30r :: Addr# chars62_r30r = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"# -- Used at the end of Bug.$wgo, in the -9223372036854775808# branch, -- therefore only used by generated Core for f, but not g! lvl_r30s :: Char lvl_r30s = case indexCharOffAddr# chars62_r30r -9223372036854775808# of v_B2 { __DEFAULT -> GHC.Types.C# v_B2 } -- Core for f Rec { Bug.$wgo :: Int# -> [Char] -> (# Char, [Char] #) Bug.$wgo = \ (ww_s2WU :: Int#) (w_s2WR :: [Char]) -> case GHC.Real.even3 of { I# y_a2QI -> -- GHC.Real.even3 == -1 case y_a2QI of { __DEFAULT -> case quotRemInt# ww_s2WU 62# of { (# ipv_a2QN, ipv1_a2QO #) -> case indexCharOffAddr# chars62_r30r ipv1_a2QO of wild2_X4 { __DEFAULT -> case <# ww_s2WU 62# of { __DEFAULT -> Bug.$wgo ipv_a2QN (GHC.Types.: @ Char (GHC.Types.C# wild2_X4) w_s2WR); 1# -> case indexCharOffAddr# chars62_r30r ww_s2WU of wild3_X1G { __DEFAULT -> (# GHC.Types.C# wild3_X1G, w_s2WR #) } } } }; 62# -> case ww_s2WU of wild2_a2QQ { __DEFAULT -> case quotRemInt# wild2_a2QQ 62# of { (# ipv_a2QT, ipv1_a2QU #) -> case indexCharOffAddr# chars62_r30r ipv1_a2QU of wild4_X4 { __DEFAULT -> case <# wild2_a2QQ 62# of { __DEFAULT -> Bug.$wgo ipv_a2QT (GHC.Types.: @ Char (GHC.Types.C# wild4_X4) w_s2WR); 1# -> case indexCharOffAddr# chars62_r30r wild2_a2QQ of wild5_X1G { __DEFAULT -> (# GHC.Types.C# wild5_X1G, w_s2WR #) } } } }; -9223372036854775808# -> case lvl_r30s of { C# v1_B2 -> (# GHC.Types.C# v1_B2, w_s2WR #) } } } } end Rec } Bug.f_go :: Int -> [Char] -> [Char] Bug.f_go = \ (w_s2WQ :: Int) (w1_s2WR :: [Char]) -> case w_s2WQ of { I# ww1_s2WU -> case Bug.$wgo ww1_s2WU w1_s2WR of { (# ww3_s2Xa, ww4_s2Xb #) -> GHC.Types.: @ Char ww3_s2Xa ww4_s2Xb } } f :: Int -> String f = \ (n__aXG :: Int) -> case n__aXG of { I# ww1_s2WU -> case Bug.$wgo ww1_s2WU (GHC.Types.[] @ Char) of { (# ww3_s2Xa, ww4_s2Xb #) -> GHC.Types.: @ Char ww3_s2Xa ww4_s2Xb } } -- Core for g Rec { Bug.$wgo1 :: Int# -> [Char] -> (# Char, [Char] #) Bug.$wgo1 = \ (ww_s2X4 :: Int#) (w_s2X1 :: [Char]) -> case GHC.Real.even3 of { I# y_a2QI -> case y_a2QI of { __DEFAULT -> case quotRemInt# ww_s2X4 62# of { (# ipv_a2QN, ipv1_a2QO #) -> case indexCharOffAddr# chars62_r30r ipv1_a2QO of wild2_X4 { __DEFAULT -> case <# ww_s2X4 62# of { __DEFAULT -> Bug.$wgo1 ipv_a2QN (GHC.Types.: @ Char (GHC.Types.C# wild2_X4) w_s2X1); 1# -> case indexCharOffAddr# chars62_r30r ww_s2X4 of wild3_XY { __DEFAULT -> (# GHC.Types.C# wild3_XY, w_s2X1 #) } } } }; 62# -> case ww_s2X4 of wild2_a2QQ { __DEFAULT -> case quotRemInt# wild2_a2QQ 62# of { (# ipv_a2QT, ipv1_a2QU #) -> case indexCharOffAddr# chars62_r30r ipv1_a2QU of wild4_X4 { __DEFAULT -> case <# wild2_a2QQ 62# of { __DEFAULT -> Bug.$wgo1 ipv_a2QT (GHC.Types.: @ Char (GHC.Types.C# wild4_X4) w_s2X1); 1# -> case indexCharOffAddr# chars62_r30r wild2_a2QQ of wild5_XY { __DEFAULT -> (# GHC.Types.C# wild5_XY, w_s2X1 #) } } } }; -9223372036854775808# -> case GHC.Real.overflowError of wild4_00 { } } } } end Rec } Bug.g_go :: Int -> [Char] -> [Char] Bug.g_go = \ (w_s2X0 :: Int) (w1_s2X1 :: [Char]) -> case w_s2X0 of { I# ww1_s2X4 -> case Bug.$wgo1 ww1_s2X4 w1_s2X1 of { (# ww3_s2Xd, ww4_s2Xe #) -> GHC.Types.: @ Char ww3_s2Xd ww4_s2Xe } } g :: Int -> String g = \ (n__a29X :: Int) -> case n__a29X of { I# ww1_s2X4 -> case Bug.$wgo1 ww1_s2X4 (GHC.Types.[] @ Char) of { (# ww3_s2Xd, ww4_s2Xe #) -> GHC.Types.: @ Char ww3_s2Xd ww4_s2Xe } } }}} Of particular interest is: {{{#!hs -- chararacter array, used by both chars62_r30r :: Addr# chars62_r30r = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"# lvl_r30s :: Char lvl_r30s = case indexCharOffAddr# chars62_r30r -9223372036854775808# of v_B2 { __DEFAULT -> GHC.Types.C# v_B2 } }}} which is only used in the Core for `f`, not `g`! We're trying to access index `minBound :: Int` of that array of chars. While this is only used when we pass `minBound` to our function, it is still wrong I think. Moreover, as [https://github.com/snowleopard/hadrian/issues/861 hadrian issue 861] showed, this can lead to... linker errors! Which got fixed by changing the implementation of `iToBase62` in Unique.hs from `f` to `g` :-) Note that when I build the same commit with the make build system, then GHC generates Core close to `g`'s above for _both functions_. I tried describing some of the transformations that occur in [https://github.com/snowleopard/hadrian/issues/641#issuecomment-415881512 this comment] on hadrian's issue tracker. The gist of it is that the lack of strictness in `q` leads GHC to not spotting it early and when we inline `quotRem`/`quotRemInt` and start floating things in/out and distributing `case ... of` branches around, we end up with a dedicated branch in the inner loop for `minBound` which actually makes use of the result of `indexAddrOffAddr#`, as you can see here: {{{#!hs -9223372036854775808# -> -- lvl_r30s is our bad value case lvl_r30s of { C# v1_B2 -> (# GHC.Types.C# v1_B2, w_s2WR #) } }}} whereas this is what this branch looks like for `g`: {{{#!hs -9223372036854775808# -> case GHC.Real.overflowError of wild4_00 { } }}} The `overflowError` is still there and GHC therefore realised that there's no point in computing anything since we always raise an overflow error in that branch. This `overflowError` just disappears in `f`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15570> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #3427: control what sort of entity a deprecated pragma applies to
by GHC 04 Sep '18

04 Sep '18

#3427: control what sort of entity a deprecated pragma applies to -------------------------------------+------------------------------------- Reporter: igloo | Owner: nineonine Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 6.10.4 Resolution: | Keywords: deprecate | warning, newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by nineonine): * owner: thoughtpolice => nineonine -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3427#comment:19> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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