[GHC] #15436: Compile-time panic, Prelude.!!: negative index

[GHC]

#15436: Compile-time panic, Prelude.!!: negative index
-------------------------------------+-------------------------------------
           Reporter:  pbrisbin       |             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:
-------------------------------------+-------------------------------------
 Here is a reproduction case:

 **ghc-repro.cabal**

 {{{
 name:           ghc-repro
 version:        0.0.0
 build-type:     Simple
 cabal-version:  >= 1.10

 library
   exposed-modules:
       Lib
   other-modules:
       Paths_ghc_repro
   hs-source-dirs:
       src
   build-depends:
       base
   default-language: Haskell2010
 }}}

 **src/Lib.hs**

 {{{#!hs
 {-# OPTIONS_GHC -v4 #-}
 module Lib where

 import GHC.Enum

 -- | At this many elements, it panics. One fewer, it works
 data USState = AL | AK | AZ | AR | CA | CO | CT | DE | FL -- | GA
              -- | HI | ID | IL | IN | IA | KS | KY | LA | ME | MD
              -- | MA | MI | MN | MS | MO | MT | NE | NV | NH | NJ
              -- | NM | NY | NC | ND | OH | OK | OR | PA | RI | SC
              -- | SD | TN | TX | UT | VT | VA | WA | WV | WI | WY
              -- | DC | PR | VI | AS | GU | MP | AA | AE | AP
     deriving (Eq, Show, Ord, Bounded, Read, Enum)

 data USStateOrIntl = International | US USState

 instance Enum USStateOrIntl where
   fromEnum International = 0
   fromEnum (US s) = 1 + fromEnum s
   enumFrom = boundedEnumFrom
   enumFromThen = boundedEnumFromThen
   toEnum 0 = International
   toEnum i = US . toEnum $ i - 1

 instance Bounded USStateOrIntl where
   minBound = International
   maxBound = US maxBound
 }}}


 **Results**:

 {{{
 ghc-repro-0.0.0: build (lib)
 Preprocessing library for ghc-repro-0.0.0..
 Building library for ghc-repro-0.0.0..
 Running phase HsPp HsSrcFile
 compile: input file src/Lib.hs
 *** Checking old interface for ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib
 (use -ddump-hi-diffs for more details):
 [1 of 2] Compiling Lib              ( src/Lib.hs, .stack-
 work/dist/x86_64-linux/Cabal-2.2.0.1/build/Lib.o )
 *** Parser [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 !!! Parser [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]: finished in 8.31
 milliseconds, allocated 17.533 megabytes
 *** Renamer/typechecker [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 !!! Renamer/typechecker [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 finished in 354.49 milliseconds, allocated 312.556 megabytes
 *** Desugar [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 Result size of Desugar (after optimization)
   = {terms: 752, types: 352, coercions: 33, joins: 1/4}
 !!! Desugar [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]: finished in
 142.79 milliseconds, allocated 226.278 megabytes
 *** Simplifier [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 Result size of Simplifier iteration=1
   = {terms: 1,222, types: 790, coercions: 143, joins: 1/3}
 Result size of Simplifier iteration=2
   = {terms: 1,219, types: 788, coercions: 126, joins: 0/1}
 Result size of Simplifier
   = {terms: 1,217, types: 786, coercions: 123, joins: 0/1}
 !!! Simplifier [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]: finished in
 374.08 milliseconds, allocated 587.256 megabytes
 *** Specialise [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 Result size of Specialise
   = {terms: 1,217, types: 786, coercions: 123, joins: 0/1}
 !!! Specialise [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]: finished in
 154.05 milliseconds, allocated 235.323 megabytes
 *** Float out(FOS {Lam = Just 0,
                    Consts = True,
                    OverSatApps = False}) [ghc-
 repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 Result size of Float out(FOS {Lam = Just 0,
                               Consts = True,
                               OverSatApps = False})
   = {terms: 1,551, types: 1,410, coercions: 123, joins: 0/0}
 !!! Float out(FOS {Lam = Just 0,
                    Consts = True,
                    OverSatApps = False}) [ghc-
 repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]: finished in 6.24 milliseconds,
 allocated 5.556 megabytes
 *** Simplifier [ghc-repro-0.0.0-K1NxTAQg5MjG2d3fZc1tOj:Lib]:
 Result size of Simplifier iteration=1
   = {terms: 1,667, types: 1,082, coercions: 123, joins: 7/19}
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-linux):
         Prelude.!!: negative index

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 The above output was produced through my normal tooling, so

 {{{
 stack build --resolver lts-12.2 --pedantic
 }}}

 To rule out stack, I was also able to reproduce the panic with plain cabal
 using this **Dockerfile**:

 {{{
 FROM haskell:8.4.3
 RUN mkdir /src
 WORKDIR /src
 COPY ghc-repro.cabal /src/ghc-repo.cabal
 COPY src/Lib.hs /src/src/Lib.hs
 RUN cabal build
 }}}

 {{{
 docker build --tag ghc-repro .
 }}}

 It still panics, but the output is different and much larger so I'll leave
 it here: https://8n1.org/13499/5c92

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15436>
GHC <http://www.haskell.org/ghc/>
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