I believe it was explicitly defined to support this use case, since the extra lifetime it is added on top of the regular "temporary lasts until the end of the f

I believe it was explicitly defined to support this use case, since the extra lifetime it is added on top of the regular "temporary lasts until the end of the full expression in which it is created". The "full expression" clause would be sufficient for function parameters, because the expression is not finished until the entire expression has been evaluated, and this includes executing and returning from the function that received the temporary. In an indirect way though I agree: it is important for the language to make it easy to break apart complex expressions, but without this lifetime extension, I suspect it may not be possible to un-nest function calls without creating more copies (RVO may prove me wrong though - not studied the case in full).