[MacPorts] #53816: llvm-3.9 @3.9.1: error: 'future_error' is unavailable: introduced in macOS 10.8

[MacPorts]

#53816: llvm-3.9 @3.9.1: error: 'future_error' is unavailable: introduced in macOS
10.8
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  Reporter:  cullub    |      Owner:  jeremyhu
      Type:  defect    |     Status:  assigned
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:
Resolution:            |   Keywords:
      Port:  llvm-3.9  |
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Comment (by jeremyhu):

 Yep.  This is behaving correctly.  future_error doesn't exist on Lion.  If
 you want to use llvm-3.9, set your deployment target to 10.8 or higher.

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Ticket URL: <https://trac.macports.org/ticket/53816#comment:5>
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