Support String.prototype.at on strings held in untyped values (#1948)

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7403-dyn-string-at.mjs.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7403-dyn-string-at.mjs"
+ ],
+ "diags": []
+}

packages/runtime/src/scr_dyn_invoke.c

@@ -996,7 +996,20 @@ static ScrDyn *scr_dyn_invoke_impl(
       scr_str_release(piece);
       return r;
     }
-    if ((mid == DYN_M_at) || (mid == DYN_M_concat) ||
+    if (mid == DYN_M_at) {
+      /* String.prototype.at: one UTF-16 code unit, counted from the end
+       * for a negative index; undefined outside the string. */
+      double index = dyn_index_arg(args, argc, 0, 0, what);
+      if (scr_exc_pending()) return NULL;
+      double len = (double)scr_str_utf16_len(s);
+      double k = index < 0 ? len + index : index;
+      if (k < 0 || k >= len) return scr_dyn_retain(scr_dyn_undefined());
+      ScrStr *unit = scr_str_slice(s, k, k + 1);
+      ScrDyn *r = scr_dyn_new_str(unit); /* retains */
+      scr_str_release(unit);
+      return r;
+    }
+    if ((mid == DYN_M_concat) ||
         (mid == DYN_M_indexOf) || (mid == DYN_M_lastIndexOf) ||
         (mid == DYN_M_includes)) {
       if (((mid == DYN_M_indexOf) || (mid == DYN_M_lastIndexOf) ||

tests/corpus/7403-dyn-string-at.mjs

@@ -0,0 +1,9 @@
+// String.prototype.at on strings held in untyped (checked-dynamic) values,
+// as Hono's mergePath reads `base?.at(-1)`.
+const mergePath = (base, sub, ...rest) => {
+  if (rest.length) sub = mergePath(sub, ...rest);
+  return `${base?.[0] === "/" ? "" : "/"}${base}${sub === "/" ? "" : `${base?.at(-1) === "/" ? "" : "/"}${sub?.[0] === "/" ? sub.slice(1) : sub}`}`;
+};
+console.log(mergePath("/api", "/users"), mergePath("api/", "/"), mergePath("/a", "b", "/c"));
+const values = JSON.parse(`["hello", "", "a\\ud83d\\ude00b"]`);
+for (const v of values) console.log(JSON.stringify([v.at(0), v.at(-1), v.at(1), v.at(99), v.at(-99), v.at(1.7), v.at()]));

tests/harness/corpus-costs/7403-dyn-string-at.mjs.json

@@ -0,0 +1 @@
+{ "plain": 2600, "san": 15000 }