Simplifier ticks exhausted on mapM/traverse

Summary

GHC 8.8 and 8.10 runs into Simplifier ticks exhausted on a simple function running mapM over a bunch of instances of Gen from QuickCheck.

Steps to reproduce

1. Compile the following code. It can cloned from here.

   module Test.Bonanza.Arbitrary where
   
   import Test.QuickCheck (Gen)
   
   genFields :: Gen [(String, Int)]
   genFields =
     mapM
       (\(f, g) -> (f,) <$> g)
       [ ("status", genIntField),
         ("body_bytes_sent", genIntField),
         ("http_referer", genIntField),
         ("http_user_agent", genIntField),
         ("http_x_forwarded_for", genIntField),
         ("separator", genIntField),
         ("connection", genIntField),
         ("request_time", genIntField),
         ("upstream_response_time", genIntField),
         ("upstream_cache_status", genIntField),
         ("user", genIntField),
         ("zconn", genIntField),
         ("request", genIntField),
         ("proxy_protocol_addr", genIntField)
       ]
   
   genIntField :: Gen Int
   genIntField = pure 0

2. See this error:

   Simplifier ticks exhausted
     When trying UnfoldingDone w1_s1v5
     To increase the limit, use -fsimpl-tick-factor=N (default 100).
   
     If you need to increase the limit substantially, please file a
     bug report and indicate the factor you needed.
   
     If GHC was unable to complete compilation even with a very large factor
     (a thousand or more), please consult the "Known bugs or infelicities"
     section in the Users Guide before filing a report. There are a
     few situations unlikely to occur in practical programs for which
     simplifier non-termination has been judged acceptable.
   
     To see detailed counts use -ddump-simpl-stats
     Total ticks: 17534

Expected behavior

It should compile.

Other observations

As I know nothing of the GHC internals, I will drop a few things I observed:

• If I replace the mapM with sequence $ map, the problem goes away. Replacing it with traverse does not fix the problem.
• GHC 8.6.5 is able to compile this without any problems.
• I copied code for Gen from QuickCheck into the module, the problem went away. However, the problem comes back If I put the code in a separate module and import it.
• If I write a dummy implementation of Gen, the problem goes away.
• If I replace Gen with IO, the problem goes away.

Environment

• GHC version used: 8.8.4 and 8.10.2

Optional:

• Operating System: Linux/Debian-unstable
• System Architecture: amd64