Fix broken caching in guessOffsetForZone

Author: diesieben07

src/datetime.js

@@ -389,15 +389,25 @@ function normalizeUnitWithLocalWeeks(unit) {
 // This is safe for quickDT (used by local() and utc()) because we don't fill in
 // higher-order units from tsNow (as we do in fromObject, this requires that
 // offset is calculated from tsNow).
+/**
+ * @param {Zone} zone
+ * @return {number}
+ */
 function guessOffsetForZone(zone) {
-  if (!zoneOffsetGuessCache[zone]) {
-    if (zoneOffsetTs === undefined) {
-      zoneOffsetTs = Settings.now();
-    }
+  if (zoneOffsetTs === undefined) {
+    zoneOffsetTs = Settings.now();
+  }
 
-    zoneOffsetGuessCache[zone] = zone.offset(zoneOffsetTs);
+  // Do not cache anything but IANA zones, because it is not safe to do so.
+  // Guessing an offset which is not present in the zone can cause wrong results from fixOffset
+  if (zone.type !== "iana") {
+    return zone.offset(zoneOffsetTs);
+  }
+  const zoneName = zone.name;
+  if (!zoneOffsetGuessCache[zoneName]) {
+    zoneOffsetGuessCache[zoneName] = zone.offset(zoneOffsetTs);
   }
-  return zoneOffsetGuessCache[zone];
+  return zoneOffsetGuessCache[zoneName];
 }
 
 // this is a dumbed down version of fromObject() that runs about 60% faster

test/datetime/create.test.js

@@ -1,6 +1,6 @@
 /* global test expect */
 
-import { DateTime } from "../../src/luxon";
+import { DateTime, Settings } from "../../src/luxon";
 import Helpers from "../helpers";
 
 const withDefaultLocale = Helpers.withDefaultLocale,
@@ -897,3 +897,25 @@ test("private language subtags don't break unicode subtags", () => {
   expect(res.outputCalendar).toBe("islamic");
   expect(res.numberingSystem).toBe("thai");
 });
+
+test("DateTime.local works even after time zone change", () => {
+  // This test catches errors produced when guessOffsetForZone produces wildy wrong guesses
+  // This guards against a regression by broken caching in that method
+  Settings.resetCaches();
+  withDefaultZone("America/Los_Angeles", () => {
+    expect(DateTime.local(2024).year).toBe(2024);
+  });
+  withDefaultZone("America/Chicago", () => {
+    const dateTime = DateTime.local(2024, 11, 3, 0, 5, 0);
+    expect(dateTime.zoneName).toBe("America/Chicago");
+    expect(dateTime.toObject()).toEqual({
+      year: 2024,
+      month: 11,
+      day: 3,
+      hour: 0,
+      minute: 5,
+      second: 0,
+      millisecond: 0,
+    });
+  });
+});