[GHC] #16066: internal error: PAP object entered!

[GHC]

#16066: internal error: PAP object entered!
----------------------------------+----------------------------------------
        Reporter:  dnspies        |                Owner:  (none)
            Type:  bug            |               Status:  new
        Priority:  normal         |            Milestone:
       Component:  Compiler       |              Version:  8.4.4
      Resolution:                 |             Keywords:
Operating System:  iOS            |         Architecture:  Unknown/Multiple
 Type of failure:  Runtime crash  |            Test Case:
      Blocked By:                 |             Blocking:
 Related Tickets:                 |  Differential Rev(s):
       Wiki Page:                 |
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Comment (by dnspies):

 I don't know about shrinking but I was able to isolate where it's
 happening.

 It's in the call to `STM`'s `return` method here:
 https://github.com/dspies-
 leapyear/persistwrap/blob/885a079923cb3eefb24db344f5e2fddee9fab425/persistwrap/src/PersistWrap/Persistable/Insert/Utils.hs#L130

 I've added a `Monad2` typeclass and threaded it through the program in
 order to make it easy to play with the bug and get more info:
 https://github.com/dspies-
 leapyear/persistwrap/blob/e8ee02716acdd42ceea8210c9ef333c0f6c9d08d
 /persistwrap-table/src/PersistWrap/Table/Monad2.hs#L20

 When I run

 {{{
 stack test persistwrap:persistwrap-test --ta '-p "Widget.should get back"'
 }}}

 I get


 {{{
 Progress 1/2: persistwrap-0.1.0.0test/Driver.hs
   widget
     Widget
       should get back what you put in: STM returning ValueSnd (V (PV 3))
 persistwrap-test: internal error: PAP object entered!
     (GHC version 8.4.4 for x86_64_apple_darwin)
     Please report this as a GHC bug:
 http://www.haskell.org/ghc/reportabug
 }}}

 So the message ''outside'' the return is getting printed but not the one
 on the inside.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16066#comment:2>
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