[GHC] #12374: Type holes show incorrect type in GHCi

[GHC]

#12374: Type holes show incorrect type in GHCi
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           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  9479, 9091
Differential Rev(s):                 |         Wiki Page:
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 In a fresh GHCi:

 {{{
 > show _

 <interactive>:7:6: error:
     • Found hole: _h :: ()
       Or perhaps ‘_h’ is mis-spelled, or not in scope
     • In the first argument of ‘show’, namely ‘_h’
       In the expression: show _h
       In an equation for ‘it’: it = show _h
     • Relevant bindings include
         it :: String (bound at <interactive>:7:1)
 }}}

 This seems wrong: the type of the hole is `_h :: Show a0 => a0`, not `_h
 :: ()`.

 On a related note, should this be reporting two errors (one for the hole,
 one for the constraint) as in #9479?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12374>
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