[GHC] #14858: Typed hole subtitution search fails in the REPL

[GHC]

#14858: Typed hole subtitution search fails in the REPL
-------------------------------------+-------------------------------------
        Reporter:  paf31             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.4.1-alpha3
  checker)                           |             Keywords:  typed holes
      Resolution:                    |  substitutions
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Probably type-class defaulting is taking place, in an effort to make
 things run.  Eg.
 {{{
 ghci> 3+4
 }}}
 We need type-class defaulting to choose `Integer` (say) as the type to use
 for the evaluation.

 Hmm.  Maybe we should NOT do any type-class defaulting if there are
 insoluble errors (like out of scope variables).  You could try this.  In
 `TcSimplify.simpl_top` we see
 {{{
 simpl_top wanteds
   = do { wc_first_go <- nestTcS (solveWantedsAndDrop wanteds)
                             -- This is where the main work happens
        ; try_tyvar_defaulting wc_first_go }
 }}}
 You could try doing `try_tyvar_defaulting` only if `insolubleWC
 wc_first_go` is False.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14858#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler