Am 09.06.2022 um 09:55 schrieb Boris Kolpackov via SG15:
> Gabriel Dos Reis via SG15 <sg15_at_[hidden]> writes:
>
>> How would a program observe that an implementation partition isn't
>> imported but is relevant to the behavior of the entire program?
> The implementation partition can register itself (as part of the
> static initialization) in some registry.
>
> For example, the implementation partition may include a unit test
> (because that's the only way to access the private parts) which
> is registered with the test harness.

The standard is totally silent about module partitions [module.unit]/3
that are also module implementation units [module.unit]/2, and which are
not required to be reachable [module.reach]/2 because they are not part
of the interface dependency [module.import]/10 graph of a module. They
may or may not have semantic effects.

If you want the behaviour from your example be portable you need to form
an interface dependency to that partition.