docs(docs-infra): extract call signature return type.

fixes #66799

packages/compiler-cli/src/ngtsc/docs/src/function_extractor.ts

@@ -152,12 +152,23 @@ function extractReturnType(signature: ts.Signature, typeChecker: ts.TypeChecker)
     return signature.declaration.type.getText();
   }
 
-  return typeChecker.typeToString(
-    typeChecker.getReturnTypeOfSignature(signature),
+  const returnType = typeChecker.getReturnTypeOfSignature(signature);
+  const returnTypeString = typeChecker.typeToString(
+    returnType,
     undefined,
     // This ensures that e.g. `T | undefined` is not reduced to `T`.
     ts.TypeFormatFlags.NoTypeReduction | ts.TypeFormatFlags.NoTruncation,
   );
+
+  if (
+    returnTypeString === 'any' &&
+    signature.declaration?.type &&
+    signature.declaration.type.kind !== ts.SyntaxKind.AnyKeyword
+  ) {
+    return signature.declaration.type.getText();
+  }
+
+  return returnTypeString;
 }
 
 /** Finds the implementation of the given function declaration overload signature. */

packages/compiler-cli/test/ngtsc/doc_extraction/interface_doc_extraction_spec.ts

@@ -8,7 +8,6 @@
 
 import {DocEntry} from '../../../src/ngtsc/docs';
 import {
-  ClassEntry,
   EntryType,
   InterfaceEntry,
   MemberTags,
@@ -382,5 +381,29 @@ runInEachFileSystem(() => {
       expect(ageSetterEntry.type).toBe('number');
       expect(ageSetterEntry.memberTags).toContain(MemberTags.Inherited);
     });
+
+    it('should extract return type for call signature', () => {
+      env.write(
+        'index.ts',
+        `
+          export interface AsyncValidatorFn {
+            (
+              control: AbstractControl,
+            ): Promise<ValidationErrors | null> | Observable<ValidationErrors | null>;
+          }
+        `,
+      );
+
+      const docs: DocEntry[] = env.driveDocsExtraction('index.ts');
+      const interfaceEntry = docs[0] as InterfaceEntry;
+      expect(interfaceEntry.members.length).toBe(1);
+
+      const methodEntry = interfaceEntry.members[0] as MethodEntry;
+      expect(methodEntry.memberType).toBe(MemberType.Method);
+      expect(methodEntry.name).toBe('');
+      expect(methodEntry.implementation.returnType).toBe(
+        'Promise<ValidationErrors | null> | Observable<ValidationErrors | null>',
+      );
+    });
   });
 });