- \*Update 2\*\*: repro case with fewer dependencies: Broken.hs: ```haskell {-# LANGUAGE OverloadedStrings #-} module Broken (breaks) where import Data.ByteString import Data.ByteString.Builder import Data.ByteString.Lazy (toStrict) import Data.String (IsString(..)) newtype Query = Query ByteString toByteString :: Builder -> ByteString toByteString x = toStrict (toLazyByteString x) instance IsString Query where fromString = Query . toByteString . stringUtf8 breaks :: [(Query, Query)] breaks = [ ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") ] ``` broken.cabal: ``` name: broken version: 0.1.0.0 build-type: Simple cabal-version: >=1.10 library exposed-modules: Broken other-extensions: OverloadedStrings build-depends: base , bytestring , bytestring-builder default-language: Haskell2010 ``` Errors with: ``` ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): Simplifier ticks exhausted When trying UnfoldingDone pokeN_a1Db To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed To see detailed counts use -ddump-simpl-stats Total ticks: 22484 ``` ---- ---- - \*Update 1\*\*: I've been able to provide a much simpler test case for this error: Broken.hs: ```haskell {-# LANGUAGE OverloadedStrings #-} module Broken (breaks) where import Database.PostgreSQL.Simple.Types (Query) breaks :: [(Query, Query)] breaks = [ ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") , ("query", "query") ] ``` broken.cabal: ``` name: broken version: 0.1.0.0 build-type: Simple cabal-version: >=1.10 library exposed-modules: Broken other-extensions: OverloadedStrings build-depends: base -- >=4.9 && <4.10 , postgresql-simple -- >=0.5 && <0.6 default-language: Haskell2010 ``` `cabal install broken.cabal` results in: ``` ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): Simplifier ticks exhausted When trying UnfoldingDone ord To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed To see detailed counts use -ddump-simpl-stats Total ticks: 20731 Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug ``` ---- ---- - \*Older info:\*\* \~\~This is unfortunately a closed-source codebase I'm experiencing this with, but we get a GHC panic with a small/not-doing-anything-crazy codebase:\~\~ ``` ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): Simplifier ticks exhausted When trying RuleFired Class op return To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed To see detailed counts use -ddump-simpl-stats Total ticks: 118123 ``` When upping to `-fsimpl-tick-factor=150`, the error is a little different: ``` ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): Simplifier ticks exhausted When trying UnfoldingDone $ To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed To see detailed counts use -ddump-simpl-stats Total ticks: 177190 ``` Upping to 200 makes the issue go away. We're building with `ghc-options: -fhpc` Before each build, we remove all .tix files and the .hpc directory At the point this error fires, we're compiling the 6th module of 9. The first 6 modules to compile only have a total of 635 lines of code. To address issues I've seen in other similar tickets: > - There are no recursive module imports > - We don't use any `{-# INLINE #-}` or similar pragmas > - There is no Template Haskell other than a `makeLenses ''App` for a small Snaplet. > - We don't have any "very"/exponentially recursive code > - We don't use any unboxed tuples (there \*is\* ST code in a module that's compiled, but not the one ghc panics on) > - We don't use type families > - We don't use TypeRep or Typeable > - We don't use Generic When this error occurs, it fails on a module which has very little code in it. It's mainly a list of \~200-300 Query\[0\] values, using OverloadedStrings. I notice Query's `mappend`, which we use, is `INLINE`d. Please let me know if I can provide more detail! \[0\] http://hackage.haskell.org/package/postgresql-simple-0.5.3.0/docs/Database-PostgreSQL-Simple-Types.html\#t:Query