fix(core): allow proper type inference when `ngFor` is used with a `trackBy` function (#42692)

In #41995 the type of `TrackByFunction` was changed such that the
declaration of a `trackBy` function did not cause the item type to be
widened to the `trackBy`'s item type, which may be a supertype of the
iterated type. This has introduced situations where the template type
checker is now reporting errors for cases where a `trackBy` function is
no longer assignable to `TrackByFunction`.

This commit fixes the error by also including the item type `T` in
addition to the constrained type parameter `U`, allowing TypeScript to
infer an appropriate `T`.

Fixes #42609

PR Close #42692

goldens/public-api/core/core.md

@@ -1258,7 +1258,7 @@ export class TestabilityRegistry {
 // @public
 export interface TrackByFunction<T> {
     // (undocumented)
-    <U extends T>(index: number, item: U): any;
+    <U extends T>(index: number, item: T & U): any;
 }
 
 // @public

packages/compiler-cli/src/ngtsc/testing/fake_core/index.ts

@@ -116,5 +116,5 @@ export interface OnDestroy {
 }
 
 export interface TrackByFunction<T> {
-  <U extends T>(index: number, item: U): any;
+  <U extends T>(index: number, item: T&U): any;
 }

packages/compiler-cli/test/ngtsc/template_typecheck_spec.ts

@@ -1086,6 +1086,78 @@ export declare class AnimationEvent {
       env.driveMain();
     });
 
+    // https://github.com/angular/angular/issues/42609
+    it('should accept NgFor iteration when trackBy is used with an `any` array', () => {
+      env.tsconfig({strictTemplates: true});
+      env.write('test.ts', `
+        import {CommonModule} from '@angular/common';
+        import {Component, NgModule} from '@angular/core';
+
+        interface ItemType {
+          id: string;
+        }
+
+        @Component({
+          selector: 'test',
+          template: '<div *ngFor="let item of anyList; trackBy: trackByBase">{{item.name}}</div>',
+        })
+        class TestCmp {
+          anyList!: any[];
+
+          trackByBase(index: number, item: ItemType): string {
+            return item.id;
+          }
+        }
+
+        @NgModule({
+          declarations: [TestCmp],
+          imports: [CommonModule],
+        })
+        class Module {}
+    `);
+
+      env.driveMain();
+    });
+
+    it('should reject NgFor iteration when trackBy is incompatible with item type', () => {
+      env.tsconfig({strictTemplates: true});
+      env.write('test.ts', `
+        import {CommonModule} from '@angular/common';
+        import {Component, NgModule} from '@angular/core';
+
+        interface ItemType {
+          id: string;
+        }
+
+        interface UnrelatedType {
+          name: string;
+        }
+
+        @Component({
+          selector: 'test',
+          template: '<div *ngFor="let item of unrelatedList; trackBy: trackByBase">{{item.name}}</div>',
+        })
+        class TestCmp {
+          unrelatedList!: UnrelatedType[];
+
+          trackByBase(index: number, item: ItemType): string {
+            return item.id;
+          }
+        }
+
+        @NgModule({
+          declarations: [TestCmp],
+          imports: [CommonModule],
+        })
+        class Module {}
+    `);
+
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(1);
+      expect(diags[0].messageText)
+          .toContain(`is not assignable to type 'TrackByFunction<UnrelatedType>'.`);
+    });
+
     it('should infer the context of NgFor', () => {
       env.tsconfig({strictTemplates: true});
       env.write('test.ts', `

packages/core/src/change_detection/differs/iterable_differs.ts

@@ -167,7 +167,7 @@ export interface TrackByFunction<T> {
    * @param index The index of the item within the iterable.
    * @param item The item in the iterable.
    */
-  <U extends T>(index: number, item: U): any;
+  <U extends T>(index: number, item: T&U): any;
 }
 
 /**