[MacPorts] #15712: Add versions to platforms

[MacPorts]

#15712: Add versions to platforms
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  Reporter:  raimue       |      Owner:  larryv
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:  MacPorts 2.6.0
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by raimue):

 Not sure about having both `-whitelist` and `-blacklist`. What would be
 the result if the expressions overlap? And yes, the meaning of the
 "negated" comparison of the version number could indeed become a problem.

 I propose the following: every supported platform should be added
 explicitly to `platforms`. Each entry specifies requirements. A plain
 platform name means any version of this platform. Specifying `{darwin >=
 11}` implicitly assumes that `{darwin < 11}` is not supported, but does
 not imply anything for any other platform. To express that a port will
 never work on a particular platform, we could use `platforms !linux`. If
 there is no entry for the current platform, the user will see a warning,
 but the build continues.

   * a) The default is empty. The option would still be mandatory in a
 Portfile as it is now.
   * b) Make the default `platforms darwin` on all platforms. The option is
 not mandatory and would not be required in almost all Portfiles.

 Realistically, we are only writing Portfiles for darwin/macOS, so b) would
 be a sane assumption in my opinion.

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Ticket URL: <https://trac.macports.org/ticket/15712#comment:20>
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