fix(common): make sure the plural category exists (#13169)

fixes #12379

modules/@angular/common/src/localization.ts

@@ -23,9 +23,23 @@ export abstract class NgLocalization { abstract getPluralCategory(value: any): s
  */
 export function getPluralCategory(
     value: number, cases: string[], ngLocalization: NgLocalization): string {
-  const nbCase = `=${value}`;
+  let key = `=${value}`;
 
-  return cases.indexOf(nbCase) > -1 ? nbCase : ngLocalization.getPluralCategory(value);
+  if (cases.indexOf(key) > -1) {
+    return key;
+  }
+
+  key = ngLocalization.getPluralCategory(value);
+
+  if (cases.indexOf(key) > -1) {
+    return key;
+  }
+
+  if (cases.indexOf('other') > -1) {
+    return 'other';
+  }
+
+  throw new Error(`No plural message found for value "${value}"`);
 }
 
 /**

modules/@angular/common/test/localization_spec.ts

@@ -7,13 +7,10 @@
  */
 
 import {LOCALE_ID} from '@angular/core';
-import {TestBed} from '@angular/core/testing';
+import {TestBed, inject} from '@angular/core/testing';
-import {beforeEach, describe, inject, it} from '@angular/core/testing/testing_internal';
-import {expect} from '@angular/platform-browser/testing/matchers';
 
 import {NgLocaleLocalization, NgLocalization, getPluralCategory} from '../src/localization';
 
-
 export function main() {
   describe('l10n', () => {
 
@@ -141,6 +138,23 @@ export function main() {
         expect(getPluralCategory(5, ['one', 'other', '=5'], l10n)).toEqual('=5');
         expect(getPluralCategory(6, ['one', 'other', '=5'], l10n)).toEqual('other');
       });
+
+      it('should fallback to other when the case is not present', () => {
+        const l10n = new NgLocaleLocalization('ro');
+        expect(getPluralCategory(1, ['one', 'other'], l10n)).toEqual('one');
+        // 2 -> 'few'
+        expect(getPluralCategory(2, ['one', 'other'], l10n)).toEqual('other');
+      });
+
+      describe('errors', () => {
+        it('should report an error when the "other" category is not present', () => {
+          expect(() => {
+            const l10n = new NgLocaleLocalization('ro');
+            // 2 -> 'few'
+            getPluralCategory(2, ['one'], l10n);
+          }).toThrowError('No plural message found for value "2"');
+        });
+      });
     });
   });
 }