Strengthen test of merge/drop function interaction.

src/cache/inmemory/__tests__/policies.ts

@@ -1873,6 +1873,32 @@ describe("type policies", function () {
           }
         `;
 
+        const deleteQuery = gql`
+          query {
+            todoList {
+              tasks(deleteOnMerge: true) {
+                taskID
+                text
+              }
+            }
+          }
+        `;
+
+        const deleteData = {
+          todoList: {
+            __typename: "ToDoList",
+            tasks: [],
+          },
+        };
+
+        // This write will cause the merge function to return undefined, but
+        // since the field is already undefined, the undefined return from the
+        // merge function should not trigger the drop function.
+        cache.writeQuery({
+          query: deleteQuery,
+          data: deleteData,
+        });
+
         cache.writeQuery({
           query,
           data: {
@@ -1903,23 +1929,11 @@ describe("type policies", function () {
 
         expect(cache.extract()).toMatchSnapshot();
 
+        // Since the ToDoList.tasks field has data now, this deletion should
+        // trigger the drop function, unlike the last time we used deleteQuery.
         cache.writeQuery({
-          query: gql`
-            query {
-              todoList {
-                tasks(deleteOnMerge: true) {
-                  taskID
-                  text
-                }
-              }
-            }
-          `,
-          data: {
-            todoList: {
-              __typename: "ToDoList",
-              tasks: [],
-            },
-          },
+          query: deleteQuery,
+          data: deleteData,
         });
       });
     });