Fix bug where a selection from a fragment may return that field in th…

…e parent query

src/core/__tests__/masking.test.ts

@@ -106,6 +106,47 @@ describe("maskOperation", () => {
     expect(data).toEqual({ user: { __typename: "User", id: 1 } });
   });
 
+  test("masks fragments from nested objects when query gets fields from same object", () => {
+    const query = gql`
+      query {
+        user {
+          __typename
+          profile {
+            __typename
+            id
+          }
+          ...UserFields
+        }
+      }
+
+      fragment UserFields on User {
+        profile {
+          __typename
+          id
+          fullName
+        }
+      }
+    `;
+
+    const data = maskOperation(
+      deepFreeze({
+        user: {
+          __typename: "User",
+          profile: { __typename: "Profile", id: "1", fullName: "Test User" },
+        },
+      }),
+      query,
+      createFragmentMatcher(new InMemoryCache())
+    );
+
+    expect(data).toEqual({
+      user: {
+        __typename: "User",
+        profile: { __typename: "Profile", id: "1" },
+      },
+    });
+  });
+
   test("deep freezes the masked result if the original data is frozen", () => {
     const query = gql`
       query {

src/core/masking.ts

@@ -149,13 +149,23 @@ function maskSelectionSet(
           memo[keyName] = data[keyName];
 
           if (childSelectionSet) {
-            const [masked, childChanged] = maskSelectionSet(
+            let [masked, childChanged] = maskSelectionSet(
               data[keyName],
               childSelectionSet,
               context,
               __DEV__ ? `${path || ""}.${keyName}` : void 0
             );
 
+            // This check prevents cases where masked fields may accidentally be
+            // returned as part of this object when the fragment also selects
+            // additional fields from the same child selection.
+            if (
+              !childChanged &&
+              Object.keys(masked).length !== Object.keys(data[keyName]).length
+            ) {
+              childChanged = true;
+            }
+
             if (childChanged) {
               memo[keyName] = masked;
               changed = true;