[GHC] #10184: Coercible solver incomplete with recursive newtypes

23 Mar 2015

      #10184: Coercible solver incomplete with recursive newtypes
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              Reporter:  goldfire    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  low         |         Milestone:  ⊥
             Component:  Compiler    |           Version:  7.10.1-rc3
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 The following module fails to compile due to infinite recursion:

 {{{
 import Data.Coerce

 newtype Bar a = Bar (Either a (Bar a))
 newtype Age = MkAge Int

 x :: Bar Age
 x = coerce (Bar (Left (5 :: Int)))
 }}}

 It tries to coerce `Bar Int` to `Bar Age`. This is clearly doable, via
 tyconapp decomposition. But we don't find it. Before worrying too much
 about it, though, we'll wait for a real example.

 Do speak up if this problem is ruining your day!

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10184
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