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

[GHC]

#10814: Eliminate some ambiguity for IsString [a]
-------------------------------------+-------------------------------------
              Reporter:  dolio       |             Owner:  ekmett
                  Type:  task        |            Status:  new
              Priority:  normal      |         Milestone:  7.12.1
             Component:  Core        |           Version:  7.10.2
  Libraries                          |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 The libraries list resolved to make a modification to the `String`
 instance for `IsString` to resolve certain ambiguous cases. One example
 is:

 {{{#!hs
 "hello" ++ " world"
 }}}

 This case can be resolved by making the instance:

 {{{#!hs
 instance (a ~ Char) => IsString [a] where ...
 }}}

 rather than:

 {{{#!hs
 instance IsString [Char] where ...
 }}}

 as the former matches only based on the head being `[a]`, and subsequently
 determines `a ~ Char`, rather than only matching when other factors cause
 the head to be fully resolved to `[Char]`.

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