[MacPorts] #59427: mono 6.0.0.319_0 Building Error on 10.15

[MacPorts]

#59427: mono 6.0.0.319_0 Building Error on 10.15
-----------------------+----------------------
  Reporter:  fuzunspm  |      Owner:  (none)
      Type:  defect    |     Status:  new
  Priority:  Normal    |  Milestone:
 Component:  ports     |    Version:  2.6.2
Resolution:            |   Keywords:  catalina
      Port:  mono      |
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Comment (by cjones051073):

 Someone (maintainer?) will need to investigate the appropriate fix for the
 port and to deploy it. If you are lucky, it might be enough to blacklist
 the Xcode 11 clang compiler (and thus fallback to MacPorts clang 9.0) but
 this is not always enough.

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Ticket URL: <https://trac.macports.org/ticket/59427#comment:6>
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