[MacPorts] #65269: mpi portgroup fails to select a C compiler variant if the default is disallowed

[MacPorts]

#65269: mpi portgroup fails to select a C compiler variant if the default is
disallowed
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  Reporter:  jmroot  |      Owner:  (none)
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:          |
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Comment (by jmroot):

 Note that the use of `compiler.blacklist` is just so I could demonstrate
 this on macOS 12; the same thing will happen on older OS versions if e.g.
 `compiler.cxx_standard` is set to something more recent, as sundials does.

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Ticket URL: <https://trac.macports.org/ticket/65269#comment:1>
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