[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
--------------------+--------------------
 Reporter:  jmroot  |      Owner:  (none)
     Type:  defect  |     Status:  new
 Priority:  Normal  |  Milestone:
Component:  ports   |    Version:
 Keywords:          |       Port:
--------------------+--------------------
 For example:
 {{{
 % port info sundials compiler.blacklist=clang
 sundials @5.8.0 (math, devel)
 Variants:             [+]accelerate, atlas, clang10, clang11, clang12,
 clang13,
                       clang90, debug, doc, examples, g95, gcc10, gcc11,
 gcc9,
                       [+]gfortran, [+]mpich, openblas, openmpi, universal
 …
 }}}
 One of the clang variants should be selected, but instead, `mpi.name` just
 ends up empty. The first noticeable symptom of this for users is the pre-
 configure variant check failing with a confusing message:
 {{{
 Error: active_variants: Error: invalid port depspec ''
 Error:   expecting either: port or (bin|lib|path):foo:port
 Error:  has a different Fortran variant () than the one selected, gcc11
 (via +gfortran).
 Error: Failed to configure sundials: Install  +gcc11
 }}}
 `configure.compiler` seems to be left as clang-14 (which is not supported
 by the mpi ports) due to no variant being selected.

-- 
Ticket URL: <https://trac.macports.org/ticket/65269>
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