.. an extension of certain compilers, and straight illegal in other compilers.

The only standard-compliant of doing something similar is with the following:

class vec3
{
    float my_vec[3];
public:
    [[nodiscard]] constexpr float& x(){
        return my_vec[0];
    }
    
    [[nodiscard]] constexpr float& y(){
        return my_vec[1];
    }

.....
};

But i see this approach less than ideal (writing () per member access might be a little tough). Hence, i propose the following syntax:

class vec3
{
    float my_vec[3];
public:
    using x = my_vec[0];
    using y = my_vec[1];
    ....
};

which would serve as a referencing mechanism to the specified elements of the array, similar to the previous method.