Implemented (including field width limitation):
werror("%q\n", (string)(indices(allocate(35)) + ({ 65, 127, 128, 130, 0x9f, 0xa0, 0x100, 0x2000, 0x22000, 0, 48 })));
"\0\1\2\3\4\5\6\a\b\t\n\v\f\r\16\17\20\21\22\23\24\25\26\27\30\31\32\e\34\35\36\37 !"A\177\200\202\237 \u0100\u2000\U00022000\u00000" (1) Result: 135
werror("%.19q\n", (string)(indices(allocate(35)) + ({ 65, 127, 128, 130, 0x9f, 0xa0, 0x100, 0x2000, 0x22000, 0, 48 })));
"\0\1\2\3\4\5\6\a" (2) Result: 19