#13264: GHC panic with (->) generalization branch while compiling lens -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by bgamari: @@ -40,49 +40,0 @@ - - While I haven't yet fully reduced the reproducer to a standalone module, - replacing `Control.Lens.Traversal` in a `lens` working tree is sufficient, - {{{#!hs - {-# LANGUAGE Rank2Types #-} - {-# LANGUAGE FlexibleContexts #-} - {-# LANGUAGE FlexibleInstances #-} - {-# LANGUAGE ScopedTypeVariables #-} - - module Control.Lens.Traversal where - - import Control.Category - import Control.Lens.Internal.Bazaar - import Control.Lens.Internal.Context - import Control.Lens.Internal.Indexed - import Data.Tagged - import Prelude hiding ((.),id) - - type Over p f s t a b = p a (f b) -> s -> f t - - holesOf :: forall p s t a. Conjoined p => Over p (Bazaar p a a) s t a a -> - s -> [Pretext p a a t] - holesOf l s = unTagged - ( conjoined - (Tagged $ let - f [] _ = [] - f (x:xs) g = Pretext (\xfy -> g . (:xs) <$> xfy x) : f xs (g . - (x:)) - in f (ins b) (unsafeOuts b)) - (Tagged $ let - f [] _ = [] - f (wx:xs) g = Pretext (\wxfy -> g . (:Prelude.map extract xs) <$> - cosieve wxfy wx) : f xs (g . (extract wx:)) - in f (pins b) (unsafeOuts b)) - :: Tagged (p a b) [Pretext p a a t] - ) where b = l sell s - }}} - - More specifically, - {{{ - $ git clone git://github.com/bgamari/hashable - $ git clone git://github.com/ekmett/comonad - $ git clone git://github.com/ekmett/semigroupoids - $ git clone git://github.com/ekmett/lens - $ cabal install ./comonad ./lens ./semigroupoids ./hashable --with- - ghc=`pwd`/inplace/bin/ghc-stage2 --allow-newer=base,template- - haskell,primitive,ghc-prim --disable-library-profiling -j1 --ghc- - options='-v -ddump-to-file -ddump-tc-trace' - }}} New description: While testing characterizing the performance impact of the Typeable branch (`wip/ttypeable`) against Hackage packages I have found that `lens` manages to break the `(->)` kind-generalization patch. Specifically, `TcCanonical.can_eq_nc` induces a panic by `tcRepSplitTyApp_maybe` during compilation of `Control.Lens.Traversal.holesOf`, {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.1.20170207 for x86_64-unknown-linux): tcRepSplitTyConApp_maybe ([] |> <*>_N ->_N Sym {alzj}) a_alzd[tau:5] c_alzb[tau:5] Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1188:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1192:37 in ghc:Outputable pprPanic, called at compiler/typecheck/TcType.hs:1456:5 in ghc:TcType tcRepSplitTyConApp_maybe, called at compiler/typecheck/TcCanonical.hs:617:25 in ghc:TcCanonical }}} The last thing emitted by tc-trace is, {{{ can_eq_nc False [WD] hole{alyP} {0}:: (p_alyn[tau:5] :: TYPE p_alym[tau:5]) GHC.Prim.~# (cat_alyK[tau:6] b_alyM[tau:6] c_alyN[tau:6] :: *) nominal equality ([] |> <*>_N ->_N Sym {alzj}) a_alzd[tau:5] -> c_alzb[tau:5] p_alyn[tau:5] cat_alyK[tau:6] b_alyM[tau:6] c_alyN[tau:6] cat_alyK[tau:6] b_alyM[tau:6] c_alyN[tau:6] }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13264#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler