[GHC] #13977: ExnStr doesn't propagate "outwards"

[GHC]

#13977: ExnStr doesn't propagate "outwards"
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 I think I might see why the demand analyser drops the `x`. Consider
 `postProcessDmdEnv`,
 {{{#!hs
 postProcessDmdEnv :: DmdShell -> DmdEnv -> DmdEnv
 postProcessDmdEnv ds@(JD { sd = ss, ud = us }) env
   | Abs <- us       = emptyDmdEnv
   | Str _ _   <- ss
   , Use One _ <- us = env  -- Shell is a no-op
   | otherwise       = mapVarEnv (postProcessDmd ds) env
 }}}

 When we get here while demand-analysing the `catchRetry# @Char a`
 application in `hello` we will have the "shell" equal to `JD {sd = Str
 ExnStr (), ud = Use One ()}` (as computed as `defer_and_use` by
 `dmdAnalStar`). The environment we are post-processing is
 `{aQq-><C(S),1*C1(U)>}`. Note that the shell has an `ExnStr` which gets
 eaten by the `Str _ _ <- ss` case of `postProcessDmdEnv`, meaning we don't
 transform the environment. I haven't yet done enough reading to know
 whether this is correct but is seems suspicious.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13977#comment:1>
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