1 files changed, 107 insertions, 0 deletions
diff --git a/js/src/tests/non262/Intl/best-available-locale-from-default-locale.js b/js/src/tests/non262/Intl/best-available-locale-from-default-locale.js
new file mode 100644
index 0000000000..16a0e0348f
--- /dev/null
+++ b/js/src/tests/non262/Intl/best-available-locale-from-default-locale.js
@@ -0,0 +1,107 @@
+// |reftest| skip-if(!this.hasOwnProperty('Intl'))
+
+if (typeof getDefaultLocale === "undefined") {
+    var getDefaultLocale = SpecialPowers.Cu.getJSTestingFunctions().getDefaultLocale;
+}
+if (typeof setDefaultLocale === "undefined") {
+    var setDefaultLocale = SpecialPowers.Cu.getJSTestingFunctions().setDefaultLocale;
+}
+
+let defaultLocale = null;
+
+function withLocale(locale, fn) {
+    if (defaultLocale === null)
+        defaultLocale = getDefaultLocale();
+
+    setDefaultLocale(locale);
+    try {
+        fn();
+    } finally {
+        setDefaultLocale(defaultLocale);
+    }
+}
+
+// This test assumes Azerbaijani ("az") is a supported locale.
+const supported = Intl.Collator.supportedLocalesOf("az");
+assertEq(supported.length, 1);
+assertEq(supported[0], "az");
+
+withLocale("az", () => {
+    // Ensure the new default locale is now active.
+    assertEq(new Intl.Collator().resolvedOptions().locale, "az");
+
+    // "az" is the active default locale, so explicitly requesting "az" should succeed.
+    assertEq(new Intl.Collator("az").resolvedOptions().locale, "az");
+
+    // ICU doesn't provide a specialised "az-Cyrl" locale, so we fallback to "az".
+    assertEq(new Intl.Collator("az-Cyrl").resolvedOptions().locale, "az");
+
+    // ICU doesn't provide a specialised "az-Cyrl-AZ" locale, so we fallback to "az".
+    assertEq(new Intl.Collator("az-Cyrl-AZ").resolvedOptions().locale, "az");
+});
+
+// As demonstrated above, "az-Cyrl-AZ" normally isn't a supported Intl.Collator locale. But when
+// used as the default locale, it gets promoted to being supported, because its parent locale "az"
+// is supported and can act as a fallback.
+//
+// This works as follows:
+// We accept any default locale as long as it can be supported either explicitly or implicitly
+// through a fallback. But when we claim a default locale is supported, we also need to make sure
+// we report any parent locale as being supported. So when "az-Cyrl-AZ" is accepted as the
+// default locale, we also need to report its parent locale "az-Cyrl" as a supported locale.
+//
+// The reason we're doing this, is to make sure we aren't limiting the supported default locale to
+// the intersection of the sets of supported locales for each Intl service constructor. Also see
+// the requirements in <https://tc39.es/ecma402/#sec-internal-slots>, which state that the default
+// locale must be a member of [[AvailableLocales]] for every Intl service constructor.
+//
+// So the following statement must hold:
+//
+// ∀ Constructor ∈ IntlConstructors: DefaultLocale ∈ Constructor.[[AvailableLocales]]
+//
+// This can trivially be achieved when we restrict the default locale to:
+//
+//                 { RequestedLocale            if RequestedLocale ∈ (∩ C.[[AvailableLocales]])
+//                 {                                                  C ∈ IntlConstructors
+//                 {
+// DefaultLocale = { Fallback(RequestedLocale)  if Fallback(RequestedLocale) ∈ (∩ C.[[AvailableLocales]])
+//                 {                                                            C ∈ IntlConstructors
+//                 {
+//                 { LastDitchLocale            otherwise
+//
+// But that severely restricts the possible default locales. For example, "az-Cyrl-AZ" is supported
+// by all Intl constructors except Intl.Collator. Intl.Collator itself only provides explicit
+// support for the parent locale "az". So with the trivial solution we'd need to mark "az-Cyrl-AZ"
+// as an invalid default locale and instead use its fallback locale "az".
+//
+// So instead of that we're using the following approach:
+//
+//                 { RequestedLocale    if RequestedLocale ∈ (∩ C.[[AvailableLocales]])
+//                 {                                          C ∈ IntlConstructors
+//                 {
+// DefaultLocale = { RequestedLocale    if Fallback(RequestedLocale) ∈ (∩ C.[[AvailableLocales]])
+//                 {                                                    C ∈ IntlConstructors
+//                 {
+//                 { LastDitchLocale    otherwise
+//
+// So even when the requested default locale is only implicitly supported through a fallback, we
+// still accept it as a valid default locale.
+withLocale("az-Cyrl-AZ", () => {
+    // Ensure the new default locale is now active.
+    assertEq(new Intl.Collator().resolvedOptions().locale, "az-Cyrl-AZ");
+
+    // "az-Cyrl-AZ" is the active default locale, so explicitly requesting the parent locale
+    // "az" should succeed.
+    assertEq(new Intl.Collator("az").resolvedOptions().locale, "az");
+
+    // "az-Cyrl-AZ" is the active default locale, so explicitly requesting the parent locale
+    // "az-Cyrl" should succeed.
+    assertEq(new Intl.Collator("az-Cyrl").resolvedOptions().locale, "az-Cyrl");
+
+    // "az-Cyrl-AZ" is the active default locale, so explicitly requesting "az-Cyrl-AZ"
+    // should succeed.
+    assertEq(new Intl.Collator("az-Cyrl-AZ").resolvedOptions().locale, "az-Cyrl-AZ");
+});
+
+if (typeof reportCompare === "function")
+    reportCompare(0, 0);