[GHC] #14722: Error message points to wrong location

[GHC]

#14722: Error message points to wrong location
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by simonpj):

 * keywords:   => TypeApplications


Comment:

 You're right. This is a consequence of the fact that (currently) GHC
 reports visible-type-application errors "eagerly", as soon as it
 encounters them, discarding all pending errors.  In this case the error
 you want reported will be pending (in the constraints being gathered) but
 it never gets a chance to be reported.

 Richard: it'd be good to find a way to defer the visible-type-application
 error.  But we don't have a way to do that yet.  A `HoleCan` isn't right.
 A constraint `[W] () ~ forall a. ???` doesn't seem right because we have
 nothing to put for the `??`.  I suppose we could invent a new kind of
 deferred error constraint, perhaps generalising `HoleCan`.

 See [wiki:TypeApplication] for a list of other VTA-related tickets; I have
 not checked but I bet that some are like this one.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14722#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler