Yes, it does.
In the Lua 5.2 source code luaV_concat
:
if (!(ttisstring(top-2) || ttisnumber(top-2)) || !tostring(L, top-1)) {
if (!call_binTM(L, top-2, top-1, top-2, TM_CONCAT))
luaG_concaterror(L, top-2, top-1);
}
else if (tsvalue(top-1)->len == 0) /* second operand is empty? */
(void)tostring(L, top - 2); /* result is first operand */
else if (ttisstring(top-2) && tsvalue(top-2)->len == 0) {
setobjs2s(L, top - 2, top - 1); /* result is second op. */
}
else {
/* at least two non-empty string values; get as many as possible */
The two else if
parts are exactly doing the job of optimizing string concatenation when one of the operand is an empty string.