Implicit conversion without alias this?

[maik klein via Digitalmars-d-learn]

I have my own version of Algebraic

struct Ok(T){
    T value;
}
struct Err(E){
    E value;
}

auto ok(T)(auto ref T value){
    return Ok!T(value);
}

auto err(E)(auto ref E err){
    return Err!E(err);
}

alias Result(T, E) = Algebraic!(Ok!T, Err!E);

I have a constructor and opAssign which allows me to write

Result!(int, string) res = ok(5);

But it seems strange that I can not do the same thing to function 
returns

Result!(int, string) test(){
    return ok(5); // Error: cannot implicitly convert expression 
(ok(5)) of type Ok!int to Algebraic!(Ok!int, Err!string)
}

I can not add implicit conversion with alias this from Ok!T to 
Result!(T, ???) because "Ok" doesn't know about the error type.

That is a bit unergonomic because I always seem to need the full 
type

like

auto ok(T, E)(auto ref T value){
    return Result!(T, E)(Ok!T(value));
}

I basically try to mirror 
http://rustbyexample.com/std/result.html but I don't think that 
is possible.

Any ideas?