Unclear error message

use Test2::V0;

use threads;

threads->create(sub {
    ok 1, 'foo';
})->join;

The above script fails with Test2 must be fully loaded before you start a new thread! at a.pl line 7 thread 1.. It is not clear at all what "fully loaded" does mean, and this concept doesn't seem to exist in Test2 documentation.

It stops crashing when I either add use Test2::IPC; or call ok(1) before creating a new thread. Perhaps the error message should recommend doing that?

Start by running the provided Perl script with and without Test2::IPC or the initial ok(1) call, then locate the code that emits the "fully loaded" error. Review the Test2 documentation for the loading concept. Done means the error explains the required initialization or recommends the relevant action clearly, with coverage for the reported script.