Steps to reproduce the issue

Any of the following are equivalent, it looks like the bug can be reproduced by attempting to concretize a package that:

• defines a variant whose values are disjoint sets, and
• whose variant provides a default value, and
• the user wishes to select a variant value other than the default

$ spack spec -I "mvapich2 process_managers=slurm" # default is 'auto'
SpecError: 'slurm' or 'auto' cannot be activated along with other process managers [mvapich2, variant 'process_managers']
# in _disjoint_set_validator, `values` is ('auto', 'slurm')

$ spack spec -I "gunrock applications=bc" # default is 'all'
SpecError: select either all or individual applications [gunrock, variant 'applications']
# in _disjoint_set_validator, `values` is ('all', 'bc')

$ spack spec -I "openloops processes=all.coll" # default is 'lhc.coll'
SpecError: this variant accepts combinations of values from exactly one of the following sets '('all.coll', 'lhc.coll')' [openloops, variant 'processes'] [openloops, variant 'processes']
# in _disjoint_set_validator, `values` is ('all.coll', 'lhc.coll')
...

Error Message

Described above, a SpecError from _disjoint_set_validator in variant.py

Information on your system

• Spack: 0.16.1
• Python: 3.8.2
• Platform: linux-rhel7-broadwell
• Concretizer: clingo

Additional information

I'll take a look at the corresponding source code but it might be that the default is added unconditionally (even if a variant value is provided by the user), or perhaps it's added before the user-specified value filters through.

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output