Iterator.prototype.flatMap FORCED check misses Safari 18.6 and 26.0.x bug(s) #1538

Iterator.prototype.flatMap throws TypeError: undefined is not a function on Safari 18.6 and 26.0.x when consumed by a short-circuiting helper over arrays. With import "core-js/actual", the FORCED check sees a callable native and skips installation, so the buggy native runs.

Reproduction (throws on Safari 18.6 and 26.0.1, passes elsewhere):

[1].values().flatMap(() => [1]).find(() => true);

Cause:

Apple's Safari 26.2 release notes describe the 26.x bug: "Fixed Iterator.prototype.flatMap to properly handle iterators without a return method" (rdar://158783404). The 18.x failure is not separately documented but the same probe triggers and the same workaround resolves it — either the same bug exists undocumented in 18.x, or a different bug surfaces similarly. Either way, behavioral detection catches both.

Proposed fix:

Add a probe to the FORCED check for es.iterator.flat-map:

var FAILS_ON_INNER_RETURN = !function () {
  try {
    [1].values().flatMap(function () { return [1]; }).find(function () { return true; });
    return true;
  } catch (e) { return false; }
}();

Workaround currently in production use:

if (typeof Iterator !== "undefined" && typeof Iterator.prototype.flatMap === "function") {
  try { [1].values().flatMap(() => [1]).find(() => true); }
  catch { Reflect.deleteProperty(Iterator.prototype, "flatMap"); }
}
import "core-js/actual";

Environment:

core-js 3.49.0, Safari 18.6 and 26.0.1, Next.js 16.2 / Turbopack.