unclear compile error for struct with string template

[Oleg B via Digitalmars-d-learn]

[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?

on dmd v2.065 it worked well

code without string template parameter work well on dmd v2.066.0

[code]
void doSome(size_t N, T)( vec!(N,T) v ) { }
struct vec(size_t N, T) { T[N] data; }
void main() { doSome( vec!(3,float)([1,2,3]) ); }
[/code]