> To handle this in code written around current __toString seems pretty simple
Yes it is, but that's not what we're talking about: BC is about having perfectly fine code working in e.g. 7.0 be still working fine on 7.1 *without any change*. Right now, we have red test suites on php7.1rc2. This is the symptom of a BC break, by definition. And the issue is not the existing code we have, but the new one that is changing the behavior of the engine.
