static opCall not working?

[TC via Digitalmars-d-learn]

I tested the code from: http://dlang.org/struct.html
Section: Dynamic Initialization of Structs

struct S
{
    int a;

    static S opCall(int v)
    {
        S s;
        s.a = v;
        return s;
    }

    static S opCall(S v)
    {
        S s;
        s.a = v.a + 1;
        return s;
    }
}

S s = 3; // sets s.a to 3
S t = s; // sets t.a to 3, S.opCall(s) is not called

which does not compile (Error: cannot implicitly convert 
expression (3) of type int to S).

What is wrong here? Docs or behavior? Tested it on asm.dlang.org 
where it fails with all compiler versions.