[BUG] UFCS doesn't work with class/struct member variables

[filipsajdak]

UFCS is not triggered when a function is called as a class method on a class member variable.

main: () -> int = {
    p := std::pair(1,2);
    p.first.ufcs(); // bad! compiles to p.first.ufcs() instead of CPP2_UFCS_0(ufcs, p.first)
}

The current implementation handle only member function calls syntax on one variable. Below are some examples of what works and what is not working + what changes with #18.

main: () -> int = {
    i := 42;
    i.ufcs();              // works

    j := fun();
    j.i.ufcs();            // doesn't work

    fun().i.ufcs();        // doesn't work

    k := fun().i;
    k.ufcs();              // works

    get_i(j).ufcs();       // works with https://github.com/hsutter/cppfront/pull/18

    get_i(fun()).ufcs();   // works with https://github.com/hsutter/cppfront/pull/18

    res := int(42).ufcs(); // works with https://github.com/hsutter/cppfront/pull/18

    int(j.i).ufcs();       // works with https://github.com/hsutter/cppfront/pull/18
}

ufcs: (i:int) -> auto = {
    return i+2;
}

fun: () -> (i:int) = {
    i = 42;
    return;
}

get_i: (r:_) -> auto = {
    return r.i;
}

I know that UFCS is still a work in progress - I am reporting that to track that. I fall into that issue when working on my small project. I was returning variables using multiple return values syntax and I was trying to trigger UFCS on one of the member variables.