add failing test for discussion

Author: yaacovCR

src/validation/__tests__/FieldsOnCorrectTypeRule-test.ts

@@ -41,7 +41,7 @@ const testSchema = buildSchema(`
     meowVolume: Int
   }
 
-  union CatOrDog = Cat | Dog
+  union CatOrDog implements Pet = Cat | Dog
 
   type Human {
     name: String
@@ -81,6 +81,15 @@ describe('Validate: Fields on correct type', () => {
     `);
   });
 
+  it('Union implementing interface field selection', () => {
+    expectValid(`
+      fragment unionImplementingInterfaceFieldSelection on CatOrDog {
+        __typename
+        name
+      }
+    `);
+  });
+
   it('Aliased interface field selection', () => {
     expectValid(`
       fragment interfaceFieldSelection on Pet {