"Tick in rule" impossible error in 8.10.1-alpha2 using Megaparsec.
## Summary
A parser I'm working on that uses [Megaparsec](https://hackage.haskell.org/package/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:
```haskell
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 8.10.0.20191210:
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 `string s`.
Using no optimization or even `-O` also results in no error.
This is as far as I've gotten.
## Environment
* GHC version used: ghc-8.10.0.20191210-x86_64-deb9-linux-dwarf
issue