fix: implementation

Author: n1ru4l

__tests__/composition.spec.ts

@@ -248,7 +248,7 @@ testImplementations((api) => {
     `);
   });
 
-  test("@override(from:) no usedOverridden in case the overriden field is @external", () => {
+  test("@override(from:) no usedOverridden in case the overriden field is @external but has matching interface fields", () => {
     const result = composeServices([
       {
         name: "a",

src/supergraph/composition/object-type.ts

@@ -18,6 +18,7 @@ import { createObjectTypeNode, JoinFieldAST } from "./ast.js";
 import type { Key, MapByGraph, TypeBuilder } from "./common.js";
 import { convertToConst } from "./common.js";
 import { InterfaceTypeFieldState } from "./interface-type.js";
+import { isExternal } from "node:util/types";
 
 export function isRealExtension(
   meta: ObjectTypeStateInGraph,
@@ -687,6 +688,16 @@ export function objectTypeBuilder(): TypeBuilder<ObjectType, ObjectTypeState> {
                   );
                   const isRequired = f.required === true;
 
+                  const inGraphs =
+                    fieldNamesOfImplementedInterfaces[field.name];
+                  const hasMatchingInterfaceFieldInGraph: boolean =
+                    inGraphs && inGraphs.has(graphId);
+
+                  // Apparently we need to keep it if it has a interface definition...
+                  if (hasMatchingInterfaceFieldInGraph) {
+                    return true;
+                  }
+
                   return (
                     (isExternal && isRequired) ||
                     needsToPrintOverrideLabel ||
@@ -1014,6 +1025,10 @@ function provideUsedOverriddenValue(
   const isOverridden =
     field.override && graphNameToId(field.override) === graphId;
 
+  if (isOverridden && fieldStateInGraph.external) {
+    return false;
+  }
+
   if (
     isOverridden &&
     (isUsedAsNonExternalKey || hasMatchingInterfaceFieldInGraph)