[GHC] #12509: ghci -XSafe fails in an inscrutable way

[GHC]

#12509: ghci -XSafe  fails in an inscrutable way
-------------------------------------+-------------------------------------
        Reporter:  int-e             |                Owner:  RolandSenn
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  SafeHaskell
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  make test
                                     |  TEST=T12509
      Blocked By:                    |             Blocking:
 Related Tickets:  #13385, #14342    |  Differential Rev(s):  Gitlab Merge
       Wiki Page:                    |  Request 23
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Comment (by int-e):

 Following in RolandSenn's trail... `ghci` evaluates some code internally.
 One such piece of code comes from `ghc/GHCi/UI/Monad.hs`,
 `initInterpBuffering`, which evaluates `System.IO.hSetBuffering
 System.IO.stdin System.IO.NoBuffering` (and the same for `stdout` and
 `stderr`). This works fine without `-XSafe`, but with `-XSafe` there is a
 stricter check that requires `System.IO` to actually be imported:

 {{{
 $ ghci
 GHCi, version 8.6.1: http://www.haskell.org/ghc/  :? for help
 Prelude> System.IO.hSetBuffering System.IO.stdin System.IO.NoBuffering
 Prelude> :set -XSafe
 Prelude> System.IO.hSetBuffering System.IO.stdin System.IO.NoBuffering

 <interactive>:3:1: error:
     Not in scope: ‘System.IO.hSetBuffering’
     No module named ‘System.IO’ is imported.

 <interactive>:3:25: error:
     Not in scope: ‘System.IO.stdin’
     No module named ‘System.IO’ is imported.

 <interactive>:3:41: error:
     Not in scope: data constructor ‘System.IO.NoBuffering’
     No module named ‘System.IO’ is imported.
 Prelude> import System.IO
 Prelude System.IO> System.IO.hSetBuffering System.IO.stdin
 System.IO.NoBuffering
 Prelude System.IO>
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12509#comment:10>
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