Implicit conversion by return

[Hakan Aras via Digitalmars-d-learn]

Given this:

struct Num
{
    this(int a) {}
}
Is there any reason why this works:

Num n = 5;

but this doesnt:

Num funk()
{
    return 5;
}


I understand that I can construct it explicitely, but that gets 
annoying quickly, especially with templates.