Why is my structure template does not compile?

[Weed]

struct S( Element )
{
     Element[] data;

     this( in uint len )
     {
         data.length = len;
     }

     ref Element opIndex( in uint n )
     {
         return data[n];
     }

     ref Element opIndexAssign( in Element a, in uint n )
     {
         data[n] += a;
         return data[n];
     }

     invariant()
     {
	// If I comment out next line compilation goes smoothly:
         assert( Element.sizeof > 0 );
     }
}

void main()
{
     alias S!(double) st;
     st test = st(20);
     auto a = test[2];
     test[2] = 3;
}

compilation error:
$ dmd demostruct
Error: __result = this is not an lvalue
demostruct.d(12): Error: __result = (this.data[n]) is not an lvalue
demostruct.d(18): Error: __result = (this.data[n]) is not an lvalue
demostruct.d(29): template instance demostruct.S!(double) error 
instantiating