pureity of closures

[Stefan Koch via Digitalmars-d]

dmd infers function closures impure if impure functions are 
defined within them.
even if those are never called and can never be accessed outside 
of the closure.

Example :
int a;
void closure() pure {
  impure_function() {
    a++;
   }
}
t.d(4): Error: pure function 't.closure.impure_function' cannot 
access mutable static data 'a'

is this intended ?