Only look for file exists and read file on supported locale directories that we build

src/compiler/utilitiesPublic.ts

@@ -318,6 +318,9 @@ namespace ts {
         return getCombinedFlags(node, n => n.flags);
     }
 
+    /* @internal */
+    export const supportedLocaleDirectories = ["cs", "de", "es", "fr", "it", "ja", "ko", "pl", "pt-br", "ru", "tr", "zh-cn", "zh-tw"];
+
     /**
      * Checks to see if the locale is in the appropriate format,
      * and if it is, attempts to set the appropriate language.
@@ -326,7 +329,8 @@ namespace ts {
         locale: string,
         sys: { getExecutingFilePath(): string, resolvePath(path: string): string, fileExists(fileName: string): boolean, readFile(fileName: string): string | undefined },
         errors?: Push<Diagnostic>) {
-        const matchResult = /^([a-z]+)([_\-]([a-z]+))?$/.exec(locale.toLowerCase());
+        const lowerCaseLocale = locale.toLowerCase();
+        const matchResult = /^([a-z]+)([_\-]([a-z]+))?$/.exec(lowerCaseLocale);
 
         if (!matchResult) {
             if (errors) {
@@ -340,7 +344,7 @@ namespace ts {
 
         // First try the entire locale, then fall back to just language if that's all we have.
         // Either ways do not fail, and fallback to the English diagnostic strings.
-        if (!trySetLanguageAndTerritory(language, territory, errors)) {
+        if (contains(supportedLocaleDirectories, lowerCaseLocale) && !trySetLanguageAndTerritory(language, territory, errors)) {
             trySetLanguageAndTerritory(language, /*territory*/ undefined, errors);
         }
 

src/testRunner/unittests/publicApi.ts

@@ -124,3 +124,30 @@ describe("unittests:: Public APIs:: getTypeAtLocation", () => {
         assert.equal(type.flags, ts.TypeFlags.Any);
     });
 });
+
+describe("unittests:: Public APIs:: validateLocaleAndSetLanguage", () => {
+    let savedUILocale: string | undefined;
+    beforeEach(() => savedUILocale = ts.getUILocale());
+    afterEach(() => ts.setUILocale(savedUILocale));
+
+    function verifyValidateLocale(locale: string, expectedToReadFile: boolean) {
+        it(`Verifying ${locale} ${expectedToReadFile ? "reads" : "does not read"} file`, () => {
+            const errors: ts.Diagnostic[] = [];
+            ts.validateLocaleAndSetLanguage(locale, {
+                getExecutingFilePath: () => "/tsc.js",
+                resolvePath: ts.identity,
+                fileExists: fileName => {
+                    assert.isTrue(expectedToReadFile, `Locale : ${locale} ${expectedToReadFile ? "should" : "should not"} check if ${fileName} exists.`);
+                    return expectedToReadFile;
+                },
+                readFile: fileName => {
+                    assert.isTrue(expectedToReadFile, `Locale : ${locale} ${expectedToReadFile ? "should" : "should not"} read ${fileName}.`);
+                    // Throw error here so that actual change to localized diagnostics messages doesnt take place
+                    throw new Error("cannot read file");
+                }
+            }, errors);
+        });
+    }
+    ts.supportedLocaleDirectories.forEach(locale => verifyValidateLocale(locale, /*expctedToReadFile*/ true));
+    ["en", "en-us"].forEach(locale => verifyValidateLocale(locale, /*expctedToReadFile*/ false));
+});