From: http://forum.dlang.org/post/pjgyejdfumdpupgjjajv@forum.dlang.org

This code should work, but doesn't.

template OriginalType(T)
{
    static if (is(T U == enum))
        alias OriginalType = U;
    else
        static assert(0);
}

enum E : int { F = -20 }
struct S
{
    int val;
    @trusted @property T as(T)()
    if (is(T == int) && !is(T == enum))
    {
        return cast(T)(val);
    }
    @trusted @property T as(T)()
    if (is(T == enum))
    {
        return cast(T)as!(OriginalType!T);  // L21
    }
}
void main()
{
    S val = S(-20);
    assert(val.as!int == -20);
    assert(val.as!E == E.F);    // L28
}

output:
test.d(21): Error: cannot cast this.as!(OriginalType!(T))
test.d(28): Error: template instance test.S.as!(E) error instantiating