[GHC] #10493: Inaccessible code might be accessible with newtypes and Coercible

[GHC]

#10493: Inaccessible code might be accessible with newtypes and Coercible
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  goldfire
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  7.10.3
       Component:  Compiler          |              Version:  7.10.1
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  typecheck/should_compile/T10493
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Unfortunately while this fix was quite straightforward to merge to
 `ghc-7.10`, it turns out that (I believe) we would also need merge this as
 well for the fix to be effective,
 {{{
 commit ff82387d6fe61762fe4f507e8f9799f5bdc3c43a
 Author: Richard Eisenberg <eir at cis.upenn.edu>
 Date:   Mon Jun 8 22:32:40 2015 -0400

     Decompose wanted repr. eqs. when no matchable givens.

     This is pursuant to a conversion with SPJ, where we agreed that
     the logic behind Note [Instance and Given overlap] in TcInteract
     applied to newtype decomposition for representational equality.

     There is no bug report or test case, as tickling this kind of thing
     is quite hard to do!
 }}}

 which is itself dependent upon,
 {{{
 commit 228ddb95ee137e7cef02dcfe2521233892dd61e0
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Wed May 13 12:49:13 2015 +0100

     Make the "matchable-given" check happen first

     This change makes the matchable-given check apply uniformly to
          - constraint tuples
          - natural numbers
          - Typeable
     as well as to vanilla class constraints.

     See Note [Instance and Given overlap] in TcInteract
 }}}

 which interacts with another patch not present in `ghc-7.10`,
 {{{
 commit a1275a762ec04c1159ae37199b1c8f998a5c5499
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Wed Apr 29 13:43:09 2015 +0100

     Improve improvement in the constraint solver
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10493#comment:9>
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