[GHC] #13469: -fdefer-type-errors for Backpack

[GHC]

#13469: -fdefer-type-errors for Backpack
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           Reporter:  ezyang         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:  backpack       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It would be great if `-fdefer-type-errors` worked with Backpack.  There
 are two senses in which it could work:

 1. When matching signatures, if the instantiating module didn't provide a
 required function, we could just create one out of thin air so we can
 continue building.

 2. When matching signature, if the implementation doesn't match the
 signature, we instead create a deferred type error.

 (2) is more difficult because it basically requires us to solve #12703.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13469>
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