[gentoo-user] USE flag "unsupported" for "sci-libs/hdf5"

[Dr Rainer Woitok <rainer.woitok@gmail.com>]

Greetings,

after  having decided to  globally set the "threads"  USE flag I get the
following:

These are the packages that would be merged, in order:

Calculating dependencies... done!

!!! The ebuild selected to satisfy "sci-libs/hdf5[mpi]" has unmet requirements.
- sci-libs/hdf5-1.10.5-r1::gentoo USE="fortran hl mpi threads zlib -cxx -debug -examples -szip -unsupported" ABI_X86="(64)"

  The following REQUIRED_USE flag constraints are unsatisfied:
    !unsupported? ( threads? ( !mpi !fortran !hl ) )

  The above constraints are a subset of the following complete expression:
    !unsupported? ( at-most-one-of ( cxx mpi ) threads? ( !cxx !mpi !fortran !hl ) )

(dependency required by "sci-libs/flann-1.9.1-r3::gentoo[mpi]" [installed])
...

Since USE flag "mpi" is required by at least one other package, it seems
I have exactly two options:

   - Set "-threads" for this package thus leaving everything as is,

   - set "unsupported" for this package, without really knowing what the
     consequences would be.

Asking "equery" isn't really enlighting in this case, is it?

$ equery --no-color --no-pipe uses sci-libs/hdf5
[ Legend : U - final flag setting for installation]
[        : I - package is installed with flag     ]
[ Colors : set, unset                             ]
 * Found these USE flags for sci-libs/hdf5-1.10.5-r1:
 U I
 - - cxx         : Build support for C++ (bindings, extra libraries, code
                   generation, ...)
 - - debug       : Enable extra debug codepaths, like asserts and extra output.
                   If you want to get meaningful backtraces see https://wiki.ge
                   ntoo.org/wiki/Project:Quality_Assurance/Backtraces
 - - examples    : Install examples, usually source code
 + + fortran     : Add support for fortran
 + + hl          : Enable high level API
                   (https://support.hdfgroup.org/HDF5/doc/HL/index.html) 
 + + mpi         : Add MPI (Message Passing Interface) layer to the apps that
                   support it
 - - szip        : Use the szip compression library
 + - threads     : Add threads support for various packages. Usually pthreads
 - - unsupported : Enable unsupported combinations of configuration options 
 + + zlib        : Add support for zlib (de)compression
$

Anybody having an educated guess what the risk would be?   Is it save to
set a USE flag even if its name is "unsupported"?

Sincerely,
  Rainer