Ambiguous call to overloaded function in c++ when using lambdas by rzlwatboa

class C
{
public:
void M(string t) {}
void M(function<string()> func) {}
};

void TestMethod(function<void()> func) {}

int _tmain(int argc, _TCHAR* argv[])
{
TestMethod([] () {
     C c;
     c.M([] () -> string { //compiler error C2668 ('function':ambiguous call to overloaded function M)
         return ("txt");
     });
    });
    

     C c2;
     c.M([] () -> string { // no compiler error
     return ("txt");
     });

    return 0;
}

I don't know why the first call to function M can't be compiled. I think this is a bug of the compiler