Ensure last results are reset when working with partial data

Issue #6334 exposed a problem where the `lastResult` mechanism
we use to prevent duplicate subscription notifications (when
data hasn't changed) can unintentionally block certain results
from propagating through Apollo Client. This leads to issues
like loading states not being updated properly, due to new
partial results looking similar to last results.

This commit ensures that last results are properly cleared out
when new partial results come in.

Fixes #6334.

package.json

@@ -51,7 +51,7 @@
     {
       "name": "apollo-client",
       "path": "./dist/apollo-client.cjs.min.js",
-      "maxSize": "24.1 kB"
+      "maxSize": "24.2 kB"
     }
   ],
   "peerDependencies": {

src/core/ObservableQuery.ts

@@ -201,7 +201,9 @@ export class ObservableQuery<
       }
     }
 
-    if (!partial) {
+    if (partial) {
+      this.resetLastResults();
+    } else {
       this.updateLastResult(result);
     }
 

src/react/hooks/__tests__/useQuery.test.tsx

@@ -380,6 +380,97 @@ describe('useQuery Hook', () => {
         console.error = consoleError;
       }).then(resolve, reject);
     });
+
+    itAsync('should update with proper loading state when variables change for cached queries', (resolve, reject) => {
+      const peopleQuery = gql`
+        query AllPeople($search: String!) {
+          people(search: $search) {
+            id
+            name
+          }
+        }
+      `;
+
+      const peopleData = {
+        people: [
+          { id: 1, name: "John Smith" },
+          { id: 2, name: "Sara Smith" },
+          { id: 3, name: "Budd Deey" }
+        ]
+      };
+
+      const mocks = [
+        {
+          request: { query: peopleQuery, variables: { search: '' } },
+          result: { data: peopleData },
+        },
+        {
+          request: { query: peopleQuery, variables: { search: 'z' } },
+          result: { data: { people: [] } },
+        },
+        {
+          request: { query: peopleQuery, variables: { search: 'zz' } },
+          result: { data: { people: [] } },
+        },
+      ];
+
+      let renderCount = 0;
+      const Component = () => {
+        const [search, setSearch] = useState('');
+        const { loading, data } = useQuery(peopleQuery, {
+          variables: {
+            search: search
+          }
+        });
+        switch (++renderCount) {
+          case 1:
+            expect(loading).toBeTruthy();
+            break;
+          case 2:
+            expect(loading).toBeFalsy();
+            expect(data).toEqual(peopleData);
+            setTimeout(() => setSearch('z'));
+            break;
+          case 3:
+            expect(loading).toBeTruthy();
+            break;
+          case 4:
+            expect(loading).toBeFalsy();
+            expect(data).toEqual({ people: [] });
+            setTimeout(() => setSearch(''));
+            break;
+          case 5:
+            expect(loading).toBeFalsy();
+            expect(data).toEqual(peopleData);
+            setTimeout(() => setSearch('z'));
+            break;
+          case 6:
+            expect(loading).toBeFalsy();
+            expect(data).toEqual({ people: [] });
+            setTimeout(() => setSearch('zz'));
+            break;
+          case 7:
+            expect(loading).toBeTruthy();
+            break;
+          case 8:
+            expect(loading).toBeFalsy();
+            expect(data).toEqual({ people: [] });
+            break;
+          default:
+        }
+        return null;
+      }
+
+      render(
+        <MockedProvider mocks={mocks}>
+          <Component />
+        </MockedProvider>
+      );
+
+      return wait(() => {
+        expect(renderCount).toBe(8);
+      }).then(resolve, reject);
+    });
   });
 
   describe('Polling', () => {