unions that implement interfaces HAVE fields

src/jsutils/mapValues.ts

@@ -0,0 +1,19 @@
+import type { ObjMap, ReadOnlyObjMap } from './ObjMap';
+
+/**
+ * Creates an object map from an array of `maps` with the same keys as each `map`
+ * in `maps` and values generated by running each value of `map` thru `fn`.
+ */
+export function mapValues<T, V>(
+  maps: ReadonlyArray<ReadOnlyObjMap<T>>,
+  fn: (value: T, key: string) => V,
+): ObjMap<V> {
+  const result = Object.create(null);
+
+  for (const map of maps) {
+    for (const key of Object.keys(map)) {
+      result[key] = fn(map[key], key);
+    }  
+  }
+  return result;
+}

src/type/definition.ts

@@ -7,6 +7,7 @@ import { isObjectLike } from '../jsutils/isObjectLike';
 import { keyMap } from '../jsutils/keyMap';
 import { keyValMap } from '../jsutils/keyValMap';
 import { mapValue } from '../jsutils/mapValue';
+import { mapValues } from '../jsutils/mapValues';
 import type { Maybe } from '../jsutils/Maybe';
 import type { ObjMap } from '../jsutils/ObjMap';
 import type { Path } from '../jsutils/Path';
@@ -1216,6 +1217,7 @@ export class GraphQLUnionType {
   astNode: Maybe<UnionTypeDefinitionNode>;
   extensionASTNodes: ReadonlyArray<UnionTypeExtensionNode>;
 
+  private _fields: GraphQLFieldMap<any, any> | undefined;
   private _types: ThunkReadonlyArray<GraphQLObjectType>;
   private _interfaces: ThunkReadonlyArray<GraphQLInterfaceType>;
 
@@ -1240,6 +1242,19 @@ export class GraphQLUnionType {
     return 'GraphQLUnionType';
   }
 
+  getFields(): GraphQLFieldMap<any, any> {
+    if (this._fields !== undefined) {
+      return this._fields;
+    }
+
+    this._fields = mapValues(
+      this.getInterfaces().map((iface) => iface.getFields()),
+      identityFunc,
+    );
+
+    return this._fields;
+  }
+
   getTypes(): ReadonlyArray<GraphQLObjectType> {
     if (typeof this._types === 'function') {
       this._types = this._types();

src/utilities/TypeInfo.ts

@@ -23,11 +23,9 @@ import {
   isEnumType,
   isInputObjectType,
   isInputType,
-  isInterfaceType,
   isListType,
   isObjectType,
   isOutputType,
-  isUnionType,
 } from '../type/definition';
 import type { GraphQLDirective } from '../type/directives';
 import {
@@ -321,17 +319,9 @@ function getFieldDef(
   if (name === TypeNameMetaFieldDef.name && isCompositeType(parentType)) {
     return TypeNameMetaFieldDef;
   }
-  if (isObjectType(parentType) || isInterfaceType(parentType)) {
+  if (isCompositeType(parentType)) {
     return parentType.getFields()[name];
   }
-  if (isUnionType(parentType)) {
-    for (const iface of parentType.getInterfaces()) {
-      const field = iface.getFields()[name];
-      if (field) {
-        return field;
-      }
-    }
-  }
 }
 
 /**

src/validation/__tests__/FieldsOnCorrectTypeRule-test.ts

@@ -43,13 +43,21 @@ const testSchema = buildSchema(`
 
   union CatOrDog = Cat | Dog
 
-  union ConstrainedCatOrDog implements Pet = Cat | Dog
+  interface Named {
+    name: String
+  }
 
-  type Human {
+  type Human implements Named {
     name: String
     pets: [Pet]
   }
 
+  type Alien implements Named {
+    name: String
+  }
+
+  union NamedHumanOrAlien implements Named = Human | Alien
+
   type Query {
     human: Human
   }
@@ -85,7 +93,7 @@ describe('Validate: Fields on correct type', () => {
 
   it('Union implementing interface field selection', () => {
     expectValid(`
-      fragment unionImplementingInterfaceFieldSelection on ConstrainedCatOrDog {
+      fragment unionImplementingInterfaceFieldSelection on HumanOrAlien {
         __typename
         name
       }