[MacPorts] #68569: linear_algebra PG: could we implement an option to override default variants from PG?

[MacPorts]

#68569: linear_algebra PG: could we implement an option to override default
variants from PG?
-------------------------+-------------------------------------------------
 Reporter:               |      Owner:  (none)
  barracuda156           |
     Type:  defect       |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.8.1
 Keywords:               |       Port:  linear_algebra, vecLibFort,
                         |  OpenBLAS
-------------------------+-------------------------------------------------
 The problem: PG sets default variant to `+accelerate` for older systems
 (to which, perhaps, there was/is a credible reason), but some ports do not
 build with old Accelerate (or do not support Accelerate at the moment). In
 those cases we have to use `OpenBLAS`, however PG does not presently allow
 custom default variant:
 {{{
 36-185% port lint --nitpick bpack
 Error: bpack: Variant accelerate conflicts with openblas
 Error: Unable to open port: Error evaluating variants
 }}}
 Of course, it will work to remove PG completely and just use `OpenBLAS`,
 exclusively, however it may be suboptimal for a number of reasons:
 1) It may not be known exactly for which systems Accelerate fails, so it
 is better to allow a user to choose a variant contrary to PG and portfile
 choice (what PG does allow).
 2) Accelerate support may be temporarily broken or not yet implemented,
 and it is bothersome to rewrite an implementation manually every time this
 occurs or throw/comment out chunks of code.
 3) Even if Accelerate (or OpenBLAS) builds are known to be broken with
 some port, it is beneficial – potentially – to make testing easier,
 allowing non-default builds (i.e. use PG, not hardcoding). But needed
 default may differ from one-size-fits-all present implementation.

 What does everyone thinks?

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Ticket URL: <https://trac.macports.org/ticket/68569>
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