[GHC] #10814: Eliminate some ambiguity for IsString [a]

[GHC]

#10814: Eliminate some ambiguity for IsString [a]
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        Reporter:  dolio             |                Owner:  dolio
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Core Libraries    |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1572
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"b225b234a6b11e42fef433dcd5d2a38bb4b466bf/ghc"
 b225b234/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="b225b234a6b11e42fef433dcd5d2a38bb4b466bf"
 Modify IsString String instance (fixes #10814)

 The new instance resolves to `s ~ [Char]` as soon as we know that `s ~
 [a]`, to avoid certain functions (like (++)) causing a situation where
 `a` is ambiguous and (currently) unable to be defaulted.

 Reviewers: #core_libraries_committee, hvr, austin, bgamari

 Reviewed By: hvr, bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1572

 GHC Trac Issues: #10814
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10814#comment:5>
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