[GHC] #14125: Bogus unacceptable type in foreign declaration.

[GHC]

#14125: Bogus unacceptable type in foreign declaration.
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        Reporter:  winter            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler (FFI)    |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * priority:  normal => high
 * failure:  None/Unknown => GHC rejects valid program
 * component:  Compiler => Compiler (FFI)


Comment:

 From what I can see, there are two different bugs being reported here:

 1. You can't use newtype instances as marshallable argument types. That
 is, this should compile, but doesn't:

 {{{#!hs
 {-# LANGUAGE ForeignFunctionInterface #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 import Foreign.C.String
 import Foreign.C.Types

 data UnixReturn

 data family IOErrno a
 newtype instance IOErrno UnixReturn = UnixErrno CInt

 foreign import ccall unsafe "string.h" strerror :: IOErrno UnixReturn ->
 IO CString
 }}}

    This is indeed a regression, as this code typechecks in 8.0.2 but not
 8.2.1. I'll try to figure out which commit caused this.

 2. You can't use newtypes as marshallable return types. However, I'm
 having trouble coming up with a minimal program which triggers the error
 you're claiming will happen on 8.2. I tried this:

 {{{#!hs
 {-# LANGUAGE ForeignFunctionInterface #-}
 module Bug where

 import Foreign
 import Foreign.C.Types
 import System.Posix.Types

 newtype UnixReturn a = UnixReturn a

 foreign import ccall unsafe "recv"
     c_recv :: CInt -> Ptr Word8 -> CSize -> CInt -> IO (UnixReturn CSsize)
 }}}

    But this typechecks on GHC 8.2, so there must be something that I'm
 missing.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14125#comment:2>
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