Python

Hm... I'll admit I wasn't awkward of the .__len__ function. ~~However, does this mean it's possible to write a len(x) == 0 that's diverges from not x?~~

~~If not, then the substitution is still valid (and not presumably also considers the same fundamental. If so, that's kind of silly.~~

EDIT: I missed the part of your comment about .__bool__ ... so yeah in theory you could have something where these two operations are not equivalent.

Arguably, you could just say that's pathological and invalid. Then, still have an optimized path to prefer not .__bool__() if .__len__() == 0 is the comparison you'd be making. Even with the extra interpreter check during evaluation, that would quite possibly be faster if the overhead is truly high.

EDIT 2: you'd probably need a little bit more overhead than a straight substitution anyways because you need to maintain the semantic of "if this thing is None it's not valid if the syntax was originally len(x)."