fix: fall back to the default locale when formatting with an unknown locale

Author: abhijeet117

src/impl/locale.js

@@ -263,7 +263,15 @@ class PolyDateFormatter {
 
     const intlOpts = { ...this.opts };
     intlOpts.timeZone = intlOpts.timeZone || z;
-    this.dtf = getCachedDTF(intl, intlOpts);
+
+    // Intl silently falls back to the system locale for unknown locales, so the
+    // default locale is listed as a backup for Intl to prefer over the system one.
+    // The formatter cache is keyed by the locales list, so changing
+    // Settings.defaultLocale picks up new formatters.
+    this.dtf =
+      Settings.defaultLocale && Settings.defaultLocale !== intl
+        ? getCachedDTF([intl, Settings.defaultLocale], intlOpts)
+        : getCachedDTF(intl, intlOpts);
   }
 
   format() {

test/datetime/toFormat.test.js

@@ -2,6 +2,9 @@
 
 import { DateTime } from "../../src/luxon";
 
+const Helpers = require("../helpers");
+const { withDefaultLocale } = Helpers;
+
 const dt = DateTime.fromObject(
   {
     year: 1982,
@@ -28,6 +31,23 @@ test("DateTime#toFormat accepts the locale from the DateTime or the options", ()
   expect(dt.setLocale("pt").toFormat("LLLL", { locale: "fr" })).toBe("mai");
 });
 
+test("DateTime#toFormat falls back to the default locale for unsupported locales", () => {
+  withDefaultLocale("fr", () => {
+    expect(dt.toFormat("LLLL", { locale: "zz" })).toBe("mai");
+    expect(dt.toFormat("ffff", { locale: "zz" })).toBe(dt.toFormat("ffff", { locale: "fr" }));
+  });
+});
+
+test("DateTime#toFormat picks up changes of the default locale for unsupported locales", () => {
+  withDefaultLocale("fr", () => {
+    expect(dt.toFormat("LLLL", { locale: "zz" })).toBe("mai");
+  });
+
+  withDefaultLocale("en-US", () => {
+    expect(dt.toFormat("LLLL", { locale: "zz" })).toBe("May");
+  });
+});
+
 test("DateTime#toFormat('u') returns fractional seconds", () => {
   expect(dt.toFormat("u")).toBe("123");
   expect(dt.set({ millisecond: 82 }).toFormat("u")).toBe("082");