Currently if you request pkg +example where example is a conditional
variant, and you have a pkg in the database for which the condition
did not hold (so no +example nor ~example), the solver would reuse it
regardless, not imposing +example.
The change rules out exactly one thing: variant_set without variant_value,
which in practice could only happen when not node_has_variant (i.e. when
under the current package.py rules the variant's when condition did not
trigger).
9ff5a30574