Difference between dstring and string format specifiers support. Bug?

[Vadim Lopatin via Digitalmars-d-learn]

Looks like bug.
dchar[] and wchar[] format strings support less specifiers than 
char[]

         import std.format;
         string test1 = "%02d".format(1); // works
         assert(test1 == "01");
         dstring test2 = "%d"d.format(1); // works
         assert(test2 == "1"d);
         wstring test3 = "%02d"w.format(1); // fails
         assert(test3 == "01"w);
         dstring test4 = "%02d"d.format(1); // fails
         assert(test4 == "01"d);