[Gen-l10n] Infer placeholder types on both templates and localizations (#163690)

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This fixes https://github.com/flutter/flutter/issues/163627 by trying to
the infer the type of a placeholder on both the template messages and
their localised versions.

This way, if the placeholder is being referenced in the localisation,
but the type is omitted in both the template and the localisation, the
check for if the template and the localisation have the same type will
not fail anymore.

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---------

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

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

@@ -584,7 +584,9 @@ class Message {
       return x && !y && !z || !x && y && !z || !x && !y && z || !x && !y && !z;
     }
 
-    for (final Placeholder placeholder in templatePlaceholders.values) {
+    for (final Placeholder placeholder in templatePlaceholders.values.followedBy(
+      localePlaceholders.values.expand((Map<String, Placeholder> e) => e.values),
+    )) {
       if (!atMostOneOf(placeholder.isPlural, placeholder.isDateTime, placeholder.isSelect)) {
         throw L10nException('Placeholder is used as plural/select/datetime in certain languages.');
       } else if (placeholder.isPlural) {

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

@@ -1689,6 +1689,50 @@ import 'output-localization-file_en.dart' deferred as output-localization-file_e
           expect(content, contains("String get helloWorld => 'Hello {name}'"));
         },
       );
+
+      // Regression test for https://github.com/flutter/flutter/issues/163627
+      //
+      // If placeholders have no explicit type (like `int` or `String`) set
+      // their type can be inferred.
+      //
+      // Later in the pipeline it is ensured that each locales placeholder types
+      // matches the definitions in the template.
+      //
+      // If only the types of the template had been inferred,
+      // and not for the translation there would be a mismatch:
+      // in this case `num` for count and `null` (the default), which is incompatible
+      // and `getSyntheticGeneratedFileContent` would throw an exception.
+      //
+      // This test ensures that both template and locale can be equally partially defined
+      // in the arb.
+      testWithoutContext(
+        'translation placeholder type definitions can be inferred for plurals',
+        () {
+          setupLocalizations(<String, String>{
+            'en': '''
+{
+  "helloWorld": "{count, plural, one{Hello World!} other{Hello Worlds!}}",
+  "@helloWorld": {
+    "description": "The conventional newborn programmer greeting",
+    "placeholders": {
+      "count": {}
+    }
+  }
+}''',
+            'de': '''
+{
+  "helloWorld": "{count, plural, one{Hallo Welt!} other{Hallo Welten!}}",
+  "@helloWorld": {
+    "description": "The conventional newborn programmer greeting",
+    "placeholders": {
+      "count": {}
+    }
+  }
+}''',
+          });
+          expect(getSyntheticGeneratedFileContent(locale: 'en'), isA<String>());
+        },
+      );
     });
 
     group('DateTime tests', () {
@@ -2167,7 +2211,7 @@ import 'output-localization-file_en.dart' deferred as output-localization-file_e
                 (L10nException e) => e.message,
                 'message',
                 contains(
-                  'The placeholder, springStartDate, has its "type" resource attribute set to the "null" type in locale "ja", but it is "DateTime" in the template placeholder.',
+                  'The placeholder, springStartDate, has its "type" resource attribute set to the "Object" type in locale "ja", but it is "DateTime" in the template placeholder.',
                 ),
               ),
             ),