Fix error when generating pt_BR localizations (#175832)

Fixes #138609

The original logic of splitting by underscore doesn't work for locales
like pt_BR where the part after the underscore is itself a valid locale.

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---------

Co-authored-by: Ben Konyi <bkonyi@google.com>

packages/flutter_tools/lib/src/localizations/gen_l10n_types.dart

@@ -638,40 +638,44 @@ class AppResourceBundle {
     // Look for the first instance of an ISO 639-1 language code, matching exactly.
     final String fileName = file.fileSystem.path.basenameWithoutExtension(file.path);
 
-    for (var index = 0; index < fileName.length; index += 1) {
-      // If an underscore was found, check if locale string follows.
-      if (fileName[index] == '_') {
-        // If Locale.tryParse fails, it returns null.
-        final Locale? parserResult = Locale.tryParse(fileName.substring(index + 1));
-        // If the parserResult is not an actual locale identifier, end the loop.
-        if (parserResult != null && _iso639Languages.contains(parserResult.languageCode)) {
-          // The parsed result uses dashes ('-'), but we want underscores ('_').
-          final String parserLocaleString = parserResult.toString().replaceAll('-', '_');
+    // Try to parse a locale from the filename.
+    String? fileNameLocale;
 
-          if (localeString == null) {
-            // If @@locale was not defined, use the filename locale suffix.
-            localeString = parserLocaleString;
-          } else {
-            // If the localeString was defined in @@locale and in the filename, verify to
-            // see if the parsed locale matches, throw an error if it does not. This
-            // prevents developers from confusing issues when both @@locale and
-            // "_{locale}" is specified in the filename.
-            if (localeString != parserLocaleString) {
-              throw L10nException(
-                'The locale specified in @@locale and the arb filename do not match. \n'
-                'Please make sure that they match, since this prevents any confusion \n'
-                'with which locale to use. Otherwise, specify the locale in either the \n'
-                'filename or the @@locale key only.\n'
-                'Current @@locale value: $localeString\n'
-                'Current filename extension: $parserLocaleString',
-              );
-            }
+    // First, try parsing the whole filename as a locale.
+    Locale? parserResult = Locale.tryParse(fileName);
+    if (parserResult != null && _iso639Languages.contains(parserResult.languageCode)) {
+      fileNameLocale = parserResult.toString().replaceAll('-', '_');
+    } else {
+      // If that fails, look for underscores and try parsing after each one.
+      for (var index = 0; index < fileName.length; index += 1) {
+        if (fileName[index] == '_') {
+          parserResult = Locale.tryParse(fileName.substring(index + 1));
+          if (parserResult != null && _iso639Languages.contains(parserResult.languageCode)) {
+            // The parsed result uses dashes ('-'), but we want underscores ('_').
+            fileNameLocale = parserResult.toString().replaceAll('-', '_');
+            break;
           }
-          break;
         }
       }
     }
 
+    if (localeString != null) {
+      // If @@locale is provided, check if there's a conflicting locale in the filename.
+      if (fileNameLocale != null && localeString != fileNameLocale) {
+        throw L10nException(
+          'The locale specified in @@locale and the arb filename do not match. \n'
+          'Please make sure that they match, since this prevents any confusion \n'
+          'with which locale to use. Otherwise, specify the locale in either the \n'
+          'filename or the @@locale key only.\n'
+          'Current @@locale value: $localeString\n'
+          'Current filename extension: $fileNameLocale',
+        );
+      }
+    } else {
+      // If @@locale is not provided, use the locale parsed from the filename.
+      localeString = fileNameLocale;
+    }
+
     if (localeString == null) {
       throw L10nException(
         "The following .arb file's locale could not be determined: \n"

packages/flutter_tools/test/general.shard/generate_localizations_test.dart

@@ -1210,6 +1210,39 @@ class AppLocalizationsEn extends AppLocalizations {
       },
     );
 
+    testWithoutContext('handles pt_BR locale correctly', () {
+      const ptArbFileString = '''
+{
+  "@@locale": "pt",
+  "test": "test_PT"
+}''';
+
+      const ptBrArbFileString = '''
+{
+  "@@locale": "pt_BR",
+  "test": "test_PT_BR"
+}''';
+
+      fs.currentDirectory.childDirectory('lib').childDirectory('l10n')
+        ..createSync(recursive: true)
+        ..childFile('pt.arb').writeAsStringSync(ptArbFileString)
+        ..childFile('pt_BR.arb').writeAsStringSync(ptBrArbFileString);
+
+      final generator = LocalizationsGenerator(
+        fileSystem: fs,
+        inputPathString: defaultL10nPath,
+        outputPathString: defaultL10nPath,
+        templateArbFileName: 'pt.arb',
+        outputFileString: defaultOutputFileString,
+        classNameString: defaultClassNameString,
+        projectPathString: fs.currentDirectory.path,
+        logger: logger,
+      )..loadResources();
+
+      expect(generator.supportedLocales.contains(LocaleInfo.fromString('pt')), true);
+      expect(generator.supportedLocales.contains(LocaleInfo.fromString('pt_BR')), true);
+    });
+
     testWithoutContext("throws when arb file's locale could not be determined", () {
       fs.currentDirectory.childDirectory('lib').childDirectory('l10n')
         ..createSync(recursive: true)