fix(query-planner): handle multiple __typename selections

.changeset/wise-yaks-switch.md

@@ -0,0 +1,8 @@
+---
+"@apollo/federation-internals": patch
+"@apollo/query-planner": patch
+---
+
+Fixed a bug that `__typename` with applied directives gets lost in fetch operations.
+
+The sibling typename optimization used by query planner simplifies operations by folding `__typename` selections into their sibling selections. However, that optimization does not account for directives or aliases. The bug was applying the optimization even if the `__typename` has directives on it, which caused the selection to lose its directives. Now, `__typename` with directives (or aliases) are excluded from the optimization.

internals-js/src/operations.ts

@@ -2114,10 +2114,10 @@
     // By default, we will print the selection the order in which things were added to it.
     // If __typename is selected however, we put it first. It's a detail but as __typename is a bit special it looks better,
     // and it happens to mimic prior behavior on the query plan side so it saves us from changing tests for no good reasons.
-    const isNonAliasedTypenameSelection = (s: Selection) => s.kind === 'FieldSelection' && !s.element.alias && s.element.name === typenameFieldName;
-    const typenameSelection = this._selections.find((s) => isNonAliasedTypenameSelection(s));
+    const isPlainTypenameSelection = (s: Selection) => s.kind === 'FieldSelection' && s.isPlainTypenameField();
+    const typenameSelection = this._selections.find((s) => isPlainTypenameSelection(s));
     if (typenameSelection) {
-      return [typenameSelection].concat(this.selections().filter(s => !isNonAliasedTypenameSelection(s)));
+      return [typenameSelection].concat(this.selections().filter(s => !isPlainTypenameSelection(s)));
     } else {
       return this._selections;
     }
@@ -3053,6 +3053,13 @@
     return this.element.definition.name === typenameFieldName;
   }
 
+  // Is this a plain simple __typename without any directive or alias?
+  isPlainTypenameField(): boolean {
+    return this.element.definition.name === typenameFieldName
+        && this.element.appliedDirectives.length == 0
+        && !this.element.alias;
+  }
+
   withAttachment(key: string, value: string): FieldSelection {
     const updatedField = this.element.copy();
     updatedField.addAttachment(key, value);

query-planner-js/src/__tests__/buildPlan.test.ts

@@ -5749,6 +5749,75 @@ describe('__typename handling', () => {
     `);
   });
 
+  it('preserves __typename with a directive', () => {
+    const subgraph1 = {
+      name: 'Subgraph1',
+      typeDefs: gql`
+        type Query {
+          t: T
+        }
+
+        type T @key(fields: "id") {
+          id: ID!
+        }
+      `,
+    };
+
+    const [api, queryPlanner] = composeAndCreatePlanner(subgraph1);
+    // Especially, when there are multiple __typename selections.
+    let operation = operationFromDocument(
+      api,
+      gql`
+        query ($v: Boolean!) {
+          t {
+            __typename
+            __typename @skip(if: $v)
+          }
+        }
+      `,
+    );
+
+    let plan = queryPlanner.buildQueryPlan(operation);
+    expect(plan).toMatchInlineSnapshot(`
+      QueryPlan {
+        Fetch(service: "Subgraph1") {
+          {
+            t {
+              __typename
+              __typename @skip(if: $v)
+            }
+          }
+        },
+      }
+    `);
+
+    operation = operationFromDocument(
+      api,
+      gql`
+        query ($v: Boolean!) {
+          t {
+            __typename @skip(if: $v)
+            __typename
+          }
+        }
+      `,
+    );
+
+    plan = queryPlanner.buildQueryPlan(operation);
+    expect(plan).toMatchInlineSnapshot(`
+      QueryPlan {
+        Fetch(service: "Subgraph1") {
+          {
+            t {
+              __typename
+              __typename @skip(if: $v)
+            }
+          }
+        },
+      }
+    `);
+  });
+
   it('does not needlessly consider options for __typename', () => {
     const subgraph1 = {
       name: 'Subgraph1',

query-planner-js/src/buildPlan.ts

@@ -3425,7 +3425,7 @@ export class QueryPlanner {
       if (
         !typenameSelection
         && selection.kind === 'FieldSelection'
-        && selection.element.name === typenameFieldName
+        && selection.isPlainTypenameField()
         && !parentMaybeInterfaceObject
       ) {
         // The reason we check for `!typenameSelection` is that due to aliasing, there can be more than one __typename selection