unclear compile error for struct with string template

[Oleg B via Digitalmars-d-learn]

On Monday, 25 August 2014 at 11:41:28 UTC, bearophile wrote:
> Oleg B:
>
>> [code]
>> void doSome(size_t N, T, string AS)( vec!(N,T,AS) v ) { }
>> struct vec(size_t N, T, string AS) { T[N] data; }
>> void main() { doSome( vec!(3,float,"xyz")([1,2,3]) ); }
>> [/code]
>>
>> compile with new dmd v2.066.0 and get error:
>>
>> Error: template opbin.doSome(ulong N, T, string AS)(vec!(N, T, 
>> AS) v) specialization not allowed for deduced parameter AS
>>
>> what does it mean?
>
> See also:
>
>
> struct Vec(size_t N, T, alias string AS) {
>     T[N] data;
> }
>
> void doSome(size_t N, T, alias string AS)(Vec!(N, T, AS) v) {}
>
> void main() {
>     auto v = Vec!(3, float, "xyz")([1, 2, 3]);
>     doSome(v);
> }
>
>
> (Note that in D the names of types and structs start with an 
> upper case).
>
> Bye,
> bearophile

Does it mean that I need write "alias" for all arrays in template 
parameters or only strings?