Nor should they. The behavior of a moved-from class is whatever you want it to be. It is not something that a compiler should be warning about.
For standard library objects, a moved-from class is in a "valid but unspecified state". As such, it is perfectly legal to do this:
std::vector<int> v{20, 30, 40};
std::vector<int> v2 = std::move(v);
v = std::vector<int>{50, 60, 70, 80};
clear doesn't care what the current state of the vector is; it just clears the vector. Thus it is reset to a known state. Similarly, operator= doesn't care what the current state is; it will reset it to a known state.
