[GHC] #10403: GHC crashes on a partial type signature

[GHC]

#10403: GHC crashes on a partial type signature
-------------------------------------+-------------------------------------
        Reporter:  Artyom.Kazak      |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Linux             |            Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash                              |               Test Case:  partial-
      Blocked By:                    |  sigs/should_compile/T10403
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by int-index):

 Simon, could you please add the code from the ticket to the test case
 verbatim? Your new variation does not trigger the bug because you removed
 the `app` function.

 Since you want to test both `h1 :: _` and `h2 :: _ => _`, please do not
 forget to include `add1` and `add2` so they get called. Here's code that
 triggers the bug:

 {{{
 {-# LANGUAGE PartialTypeSignatures #-}
 module T10403 where

 main :: IO ()
 main = return ()

 data I a = I a
 instance Functor I where
     fmap f (I a) = I (f a)

 newtype B t a = B a
 instance Functor (B t) where
     fmap f (B a) = B (f a)

 newtype H f = H (f ())

 app1 :: H (B t)
 app1 = h1 (H . I) (B ())

 app2 :: H (B t)
 app2 = h2 (H . I) (B ())

 h1 :: _ => _
 --h1 :: Functor m => (a -> b) -> m a -> H m
 h1 f b = (H . fmap (const ())) (fmap f b)

 h2 :: _
 --h2 :: Functor m => (a -> b) -> m a -> H m
 h2 f b = (H . fmap (const ())) (fmap f b)
 }}}

 The error message I'm getting when loading it in GHCi is:
 {{{
 GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
 λ> :l Bug.hs
 [1 of 1] Compiling T10403           ( Bug.hs, interpreted )

 Bug.hs:21:12:
     Couldn't match type ‘b’ with ‘H I’
       ‘b’ is untouchable
         inside the constraints ()
         bound by the type signature for app2 :: H (B t)
         at Bug.hs:20:9-15ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.1 for x86_64-unknown-linux):
         No skolem info: b_ao6[sk]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10403#comment:14>
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