[GHC] #13461: Panic on compile of file with a hole and nothing else (was: Panic on compile of file with a hole, importing a module with a lot of identifiers)

[GHC]

#13461: Panic on compile of file with a hole and nothing else
-------------------------------------+-------------------------------------
        Reporter:  Tritlo            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  MacOS X           |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by Tritlo:

Old description:

> So I was trying to test the performance of `validSubstitutions`, I
> generated a file with a lot of identifiers, and compiled it with my
> current GHC, `The Glorious Glasgow Haskell Compilation System, version
> 8.0.2`.
>
> The example was very simple, just:
>
> {{{
> import ManyIds
>
> test :: Int -> Int
> test = _
> }}}
>
> Where `ManyIds.hs` was generated with the python script:
> {{{
> print("module ManyIds where")
> print()
>
> for i in range(10000):
>     print("a{} :: Int -> Int".format(i))
>     print("a{} _ = {}".format(i,i))
>     print()
> }}}
>
> i.e. it has a lot of identifiers of the form
>
> {{{
> ...
> a1337 :: Int -> Int
> a1337 _ = 1337
>
> a1338 :: Int -> Int
> a1338 _ = 1338
> ...
> }}}
>
> The error I  got was:
>
> {{{
> [2 of 2] Compiling Main             ( manyIdsTest.hs, manyIdsTest.o )
> ghc: panic! (the 'impossible' happened)
>   (GHC version 8.0.2 for x86_64-apple-darwin):
>         initTc: unsolved constraints
>   WC {wc_insol = [W] __a5IO :: t_a5IN[tau:1] (CHoleCan: _)}
>
> Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
> }}}

New description:

 The example is very simple, just:

 {{{
 test :: Int -> Int
 test = _
 }}}

 which produces the panic:

 {{{
 [1 of 1] Compiling Main             ( manyIdsTest.hs, manyIdsTest.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-apple-darwin):
         initTc: unsolved constraints
   WC {wc_insol = [W] __awc :: t_awb[tau:1] (CHoleCan: _)}
 }}}

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13461#comment:1>
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