Surprising behavior of std::function move constructor in Apple clang

move_function.cc

#include <stdio.h>

#include <functional>
#include <utility>

struct Big {
    char data[1024];
};

int main(int argc, char **argv) {
    Big blob;
    // This bind will trigger small object optimization
    std::function<void()> little = std::bind([]() { printf("little\n"); });
    // This bind will not
    std::function<void()> big = std::bind([](Big const& b) {
            printf("big %c\n", b.data[0]);
        }, blob);
    auto little_moved = std::move(little);
    auto big_moved = std::move(big);

    // After move, one expects the source std::function to be empty
    // (boolean value false)
    
    printf("Little empty: %d\n", !little);
    printf("Little (moved) empty: %d\n", !little_moved);
    printf("Big empty: %d\n", !big);
    printf("Big (moved) empty: %d\n", !big_moved);

    return 0;
}