C++11 lambda can be assigned to std::function with incorrect signature

The following compiles and runs (under Apple LLVM version 6.1.0 and Visual C++ 2015):

#include <functional>
#include <iostream>

struct s { int x; };

int main(int argc, char **argv)
{
    std::function<void (s &&)> f = [](const s &p) { std::cout << p.x; };
    f(s {1});
    return 0;
}

Why doesn't the assignment std::function<void (s &&)> f = [](const s &p) { std::cout << p.x; }; generate an error? A function accepting an rvalue reference should not have the same signature as a function accepting a const lvalue reference, should it? Dropping the const from the lambda's declaration does generate an error as expected.