Template mixins has no effect

["Nordlöw" via Digitalmars-d-learn]

I'm trying to improve

https://github.com/nordlow/justd/blob/master/enums.d

through use of template mixins

http://dlang.org/template-mixin.html

to improve EnumUnion as follows:

struct EnumUnion(E...)
{
     alias OriginalType = CommonOriginalType!E;
     alias U = UnionEnum!(E);    // Wrapped Type.
     alias _value this;

     mixin template genOpAssign(uint i)
     {
         @safe pure nothrow void opAssign(E[i] e)
         {
             _value = cast(U)e;
         }
     }

     mixin template genOpCast(uint i)
     {
         @safe pure nothrow E[i] opCast(T : E[i])() const @safe 
pure nothrow {
             bool match = false;
             foreach (m; EnumMembers!(E[i])) {
                 if (m == _value) {
                     match = true;
                 }
             }
             if (!match)
                 throw new RangeError();
             return cast(E[i])_value;
         }
     }

     /* TODO: Alternative to this set of static if? */
     static if (E.length >= 1) { mixin genOpAssign!0; mixin 
genOpCast!0; }
     static if (E.length >= 2) { mixin genOpAssign!1; mixin 
genOpCast!1; }
     static if (E.length >= 3) { mixin genOpAssign!2; mixin 
genOpCast!2; }

     private U _value;           // Instance.
}

but it doesn't see to have an effect on the conversion rules:

enums.d(162,10): Error: cannot implicitly convert expression (e0) 
of type E0 to EnumUnion!(E0, E1, E2)

What is wrong?