alias c=mixin(expr); disallowed, why?

[Timothee Cour]

Is there a reason the language spec disallows this?

----
void main(){
auto a=mixin("1");//OK
alias b=a;//OK
mixin("alias c=a;");//OK
// alias c=mixin("a");//NG : Error: basic type expected, not mixin
}
----