"Tick in rule" impossible error in 8.10.1-alpha2 using Megaparsec.
A parser I'm working on that uses Megaparsec (either 7.0.5 or 8.0.0) is failing to compile under the pre-release of 8.10.1 with an "impossible" error.
Steps to reproduce
I have stripped it down to the following:
module TickInRule where import Control.Monad (void) import Text.Megaparsec import Text.Megaparsec.Char foo :: String -> Parsec () String () foo s = do manyTill (foo "a") (lookAhead $ void (string s) <|> eof) void (string s)
(This parser doesn't make much sense of course, but I've removed everything not necessary for the error.)
I get this using
ghc -O2 bad.hs:
[1 of 1] Compiling TickInRule ( bad.hs, bad.o ) ghc: panic! (the 'impossible' happened) (GHC version 220.127.116.1191210: Tick in rule src<libraries/base/GHC/Base.hs:(1448,1)-(1453,15)> src<libraries/base/Data/List/NonEmpty.hs:(187,1)-(190,51)> src<libraries/base/GHC/Base.hs:(1403,1)-(1418,26)> lvl_s3lT Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable pprPanic, called at compiler/specialise/Rules.hs:720:5 in ghc:Rules Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug
Oddly, factoring out the void part (
let v = void (string s)) makes the error go away; indeed, so does factoring out just
Using no optimization or even
-O also results in no error.
This is as far as I've gotten.
- GHC version used: ghc-18.104.22.16891210-x86_64-deb9-linux-dwarf