So, it's basically a

CHECK {
    die unless __PACKAGE__ -> can ("paint");
}
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except that the actually check is delayed till run time?

In that case, I'd rather see an interface like:

use Class:Trait qw /paint/;
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with no explicite assemble call, but have the checking done at compile time. That might be useful if you write a module that are only useful if callbacks are installed.

Abigail