Re: [GHC] #13932: GHC and GHCi dumps "panic!" ("initTc: unsolved constraints")

7 Jul 2017

      #13932: GHC and GHCi dumps "panic!" ("initTc: unsolved constraints")
-------------------------------------+-------------------------------------
        Reporter:  aiya000           |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  duplicate         |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13106            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => duplicate
 * related:   => #13106

Comment:

 Thanks for the bug report.

 This is a duplicate of #13106, and has been fixed in GHC 8.2.

 For what it's worth, this panic sometimes happens when you have out-of-
 scope identifiers. Here's the error message you get on GHC 8.2:

 {{{
 [6 of 6] Compiling Maru.Main        ( src/Maru/Main.hs, interpreted )

 src/Maru/Main.hs:42:15: error:
     • Variable not in scope:
         makeLensesWith :: t1 -> String -> Name -> DecsQ
     • Perhaps you meant ‘makeLensesA’ (line 42)
       Perhaps you want to add ‘makeLensesWith’ to the import list
       in the import of ‘Lens.Micro.TH’ (src/Maru/Main.hs:26:1-45).
    |
 42 | makeLensesA = makeLensesWith (lensRules & lensField .~ addSuffix)
    |               ^^^^^^^^^^^^^^

 src/Maru/Main.hs:42:31: error:
     • Variable not in scope: lensRules
     • Perhaps you want to add ‘lensRules’ to the import list
       in the import of ‘Lens.Micro.TH’ (src/Maru/Main.hs:26:1-45).
    |
 42 | makeLensesA = makeLensesWith (lensRules & lensField .~ addSuffix)
    |                               ^^^^^^^^^

 src/Maru/Main.hs:42:41: error:
     Variable not in scope:
       (&)
         :: t5
            -> (((Name -> [Name] -> Name -> [DefName])
                 -> f0 (Name -> [Name] -> Name -> [DefName]))
                -> Lens.Micro.TH.LensRules -> f0 Lens.Micro.TH.LensRules)
            -> t3
    |
 42 | makeLensesA = makeLensesWith (lensRules & lensField .~ addSuffix)
    |                                         ^

 src/Maru/Main.hs:42:53: error:
     • Variable not in scope:
         (.~) :: t3 -> (Name -> [Name] -> Name -> [DefName]) -> t1
     • Perhaps you meant ‘.’ (imported from Prelude)
    |
 42 | makeLensesA = makeLensesWith (lensRules & lensField .~ addSuffix)
    |                                                     ^^

 src/Maru/Main.hs:54:1: error:
     GHC stage restriction:
       ‘makeLensesA’ is used in a top-level splice, quasi-quote, or
 annotation,
       and must be imported, not defined locally
    |
 54 | makeLensesA ''CliOptions
    | ^^^^^^^^^^^
 }}}

 So ultimately, the root of the issue is that you're trying to use things
 which aren't imported (e.g., `makeLensesWith`).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13932#comment:4
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