Hi, The code below (also attached - unzip and run go.sh) triggers the GHC panic "Simplifier ticks exhausted", and I'm unsure whether I should view it as an instance of the known infelicity in the inliner (https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#bu...) My code does have a recursive datatype, but the recursion isn't contravariant, which is the case described in "Secrets of the GHC inliner" (https://www.microsoft.com/en-us/research/wp-content/uploads/2002/07/inline.p..., section 4). It's cut down from some real code where I have a mutually recursive datatype that I want to define across two modules for code structuring reasons, meaning I need a .hs-boot file. I haven't been able to reproduce it without the .hs-boot file - if I put everything in one module it stops happening. I've tried with a range of GHC versions from 8.2.x to an early version of 8.8. It happens with -O1 and not -O0, but I haven't managed to find a specific optimisation that triggers it. Is this just an instance of the known problem in a different guise, or is it something different and worth a bug report? Cheers, Ganesh T2.hs-boot ----------- module T2 where data T2 mapP_T2 :: (Int -> Int) -> T2 -> T2 T1.hs ----- module T1 where import {-# SOURCE #-} T2 data T1 = T1 T2 mapP_T1 :: (Int -> Int) -> T1 -> T1 mapP_T1 _ (T1 xs) = T1 (mapP_T2 id xs) T2.hs ----- module T2 where import T1 data T2 = T2 T1 mapP_T2 :: (Int -> Int) -> T2 -> T2 mapP_T2 f (T2 t) = T2 (mapP_T1 f t) go :: T1 -> T1 go = mapP_T1 id GHC output ---------- $ ghc --make T2.hs -O1 -fsimpl-tick-factor=1000 -ddump-simpl-stats) [...] ghc.exe: panic! (the 'impossible' happened) (GHC version 8.2.2 for x86_64-unknown-mingw32): Simplifier ticks exhausted When trying UnfoldingDone mapP_T2 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 Total ticks: 61203 24481 PreInlineUnconditionally 6121 ds_i17h 6120 f_a16p 6120 ds_d17d 6120 ds1_i17i 12241 UnfoldingDone 6121 mapP_T1 6120 mapP_T2 24481 BetaReduction 6121 ds_i17h 6120 f_a16p 6120 ds_d17d 6120 ds1_i17i