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);