Why is GCC warning me about a fallthrough even when I use [[fallthrough]]?

In the following piece of code, I use the standard [[fallthrough]] attribute from C++1z to document that a fallthrough is desired:

#include <iostream>

int main() {
    switch (0) {
        case 0:
            std::cout << "a\n";
            [[fallthrough]]
        case 1:
            std::cout << "b\n";
            break;
    }
}

With GCC 7.1, the code compiles without an error. However, the compiler still warns me about a fallthrough:

warning: this statement may fall through [-Wimplicit-fallthrough=]
    std::cout << "a\n";
    ~~~~~~~~~~^~~~~~~~

Why?