Fix supportedLocalesOf of "best fit" matcher

Only return "subset of" the requested Locales.


Bug: v8:11860
Change-Id: I917753c6f0f5dfc7a52d071febbe03abfab45b04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2946746Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75025}

src/objects/intl-objects.cc

@@ -1564,8 +1564,13 @@ std::vector<std::string> BestFitSupportedLocales(
           matcher.getBestMatchResult(desired, status);
       if (U_FAILURE(status)) continue;
       if (matched.getSupportedIndex() < 0) continue;
-      std::string bestfit =
-          matched.makeResolvedLocale(status).toLanguageTag<std::string>(status);
+
+      // The BestFitSupportedLocales abstract operation returns the *SUBSET* of
+      // the provided BCP 47 language priority list requestedLocales for which
+      // availableLocales has a matching locale when using the Best Fit Matcher
+      // algorithm. Locales appear in the same order in the returned list as in
+      // requestedLocales. The steps taken are implementation dependent.
+      std::string bestfit = desired.toLanguageTag<std::string>(status);
       if (U_FAILURE(status)) continue;
       result.push_back(bestfit);
     }

test/intl/regress-11860.js

+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony_intl_best_fit_matcher
+//
+// Test the return list is subset of the request list.
+// https://tc39.es/ecma402/#sec-bestfitsupportedlocales
+//  9.2.9 BestFitSupportedLocales ( availableLocales, requestedLocales )
+// The BestFitSupportedLocales abstract operation returns the SUBSET of the
+// provided BCP 47 language priority list requestedLocales for which
+// availableLocales has a matching locale when using the Best Fit Matcher
+// algorithm. Locales appear in the same order in the returned list as in
+// requestedLocales. The steps taken are implementation dependent.
+
+
+function assertSubarray(a, b) {
+  assertTrue(a.every(val => b.includes(val)), ['returns:', a, 'requested:', b]);
+}
+
+function verifySupportedLocalesAreSubarray(f, ll) {
+  assertSubarray(f(ll), ll);
+}
+
+verifySupportedLocalesAreSubarray(Intl.DateTimeFormat.supportedLocalesOf, ['en', 'ceb']);
+verifySupportedLocalesAreSubarray(Intl.DateTimeFormat.supportedLocalesOf, ['en', 'ceb', 'fil']);