fix(common): weaken AsyncPipe transform signature (#22169)

The AsyncPipe type signature was changed to allow
deferred creation of promises and observalbes that
is supported by the implementation by allowing
`Promise<T>|null|undefined` and by allowing
`Observable<T>|null|undefined`.

PR Close #22169

packages/common/src/pipes/async_pipe.ts

@@ -85,8 +85,8 @@ export class AsyncPipe implements OnDestroy, PipeTransform {
 
   transform<T>(obj: null): null;
   transform<T>(obj: undefined): undefined;
-  transform<T>(obj: Observable<T>): T|null;
-  transform<T>(obj: Promise<T>): T|null;
+  transform<T>(obj: Observable<T>|null|undefined): T|null;
+  transform<T>(obj: Promise<T>|null|undefined): T|null;
   transform(obj: Observable<any>|Promise<any>|null|undefined): any {
     if (!this._obj) {
       if (obj) {

packages/compiler-cli/test/diagnostics/check_types_spec.ts

@@ -620,12 +620,12 @@ describe('ng type checker', () => {
     });
   });
 
-  describe('regressions ', () => {
+  describe('core', () => {
     const a = (files: MockFiles, options: ng.AngularCompilerOptions = {}) => {
       accept(files, {fullTemplateTypeCheck: true, ...options});
     };
 
-    // #19905
+    // Regression #19905
     it('should accept an event binding', () => {
       a({
         'src/app.component.ts': '',
@@ -655,6 +655,38 @@ describe('ng type checker', () => {
     });
   });
 
+  describe('common', () => {
+    const a = (files: MockFiles, options: ng.AngularCompilerOptions = {}) => {
+      accept(files, {fullTemplateTypeCheck: true, ...options});
+    };
+
+    // Regression #19905
+    it('should accept a |undefined or |null parameter for async_pipe', () => {
+      a({
+        'src/app.component.ts': '',
+        'src/lib.ts': '',
+        'src/app.module.ts': `
+        import {NgModule, Component} from '@angular/core';
+        import {CommonModule} from '@angular/common';
+
+        @Component({
+          selector: 'comp',
+          template: '<div>{{ name | async}}</div>'
+        })
+        export class MainComp {
+          name: Promise<string>|undefined;
+        }
+
+
+        @NgModule({
+          declarations: [MainComp],
+          imports: [CommonModule]
+        })
+        export class MainModule {}`
+      });
+    });
+  });
+
   describe('with modified quickstart (fullTemplateTypeCheck: false)', () => {
     addTests({fullTemplateTypeCheck: false});
   });

tools/public_api_guard/common/common.d.ts

@@ -5,8 +5,8 @@ export declare const APP_BASE_HREF: InjectionToken<string>;
 export declare class AsyncPipe implements OnDestroy, PipeTransform {
     constructor(_ref: ChangeDetectorRef);
     ngOnDestroy(): void;
-    transform<T>(obj: Promise<T>): T | null;
-    transform<T>(obj: Observable<T>): T | null;
+    transform<T>(obj: Promise<T> | null | undefined): T | null;
+    transform<T>(obj: Observable<T> | null | undefined): T | null;
     transform<T>(obj: undefined): undefined;
     transform<T>(obj: null): null;
 }