The test() function fails when the function it runs returns a value. It then tries to determine if the value is a Promise to suggest using promise_test() instead. To do so, it checks if the returned value has an own property called "then".

This does not work correctly, as then() is defined in Promise.prototype, not Promise itself. Quick way to reproduce:

test(async t => {
}, "my test");

The harness error is "Test named "my test" inappropriately returned a value" rather than "Test named "my test" inappropriately returned a value, consider using promise_test instead".