std::map Gives Same Value When Reversing the Order Within the Key in C++

I am using std::map with key as a user-defined Struct and value as an int. Concretely, it looks something like this: map<myType, int>.

My Struct, let's call it myType, has 2 attributes, call them .a and .b, both of which are of type int.

When I do something like the following

mp[{5, 7}] = 0;
mp[{7, 5}] = 1;

and I access mp[{5, 7}], I get 1, instead of 0. This means that {.a, .b} and {.b, .a} map to the same value.

My understanding is that the root of the problem lies in the comparison operator I defined for myType.

Here is my definition of myType, with the comparison operator included:

struct myType {
    int a;
    int b;
    bool operator<(myType m) const {
        if (a == m.a) return (b <= m.b);
        else return (a <= m.a);
    }
};

How can I make {.a, .b} and {.b, .a} map to different values without using std::pair?