明示的フォワードでも駄目なんだね

gistfile1.d

void main()
{
    enum s = S(S.F(42));
    alias W!(          s        .a ) k3;    // 'a'
//  alias W!(wantValue(s        .a)) k4;    // Error: this for a needs to be type F not type A
    alias W!(          s.forward.a ) k1;    // 'a'
    alias W!(wantValue(s.forward.a)) k2;    // '42'
}

alias W!(S        .square) symA;
alias W!(S.forward.square) symB;
static assert(__traits(isSame, symA, symB));

template W(alias k) { pragma(msg, k); }

struct S
{
    struct F
    {
        int a;

        int  square(int  n) { return n*n; }
        real square(real n) { return n*n; }

        static @property int boo() { return 42; }
    }

    F forward;
    /+
    @property void forward(F fwd);
    @property F    forward(     ) { return F(a); }
    +/

    alias forward.a      a;
    alias forward.square square;
    alias forward.boo    boo;
    //alias forward this;
}

T wantValue(T)(T v) { return v; }