refactor(forms): nicer type errors

By intersecting with `object` instead of `unknown` in the primitive and
`FormControl` cases, we get TypeScript to show nicer type errors that
mention `FieldTree<...>` insetad of `() => FieldState<...>`

Author: mmalerba

goldens/public-api/forms/signals/index.api.md

@@ -181,7 +181,7 @@ export interface FieldState<TValue, TKey extends string | number = string | numb
 }
 
 // @public
-export type FieldTree<TModel, TKey extends string | number = string | number> = (() => [TModel] extends [AbstractControl] ? CompatFieldState<TModel, TKey> : FieldState<TModel, TKey>) & ([TModel] extends [AbstractControl] ? unknown : [TModel] extends [Array<infer U>] ? ReadonlyArrayLike<MaybeFieldTree<U, number>> : TModel extends Record<string, any> ? Subfields<TModel> : unknown);
+export type FieldTree<TModel, TKey extends string | number = string | number> = (() => [TModel] extends [AbstractControl] ? CompatFieldState<TModel, TKey> : FieldState<TModel, TKey>) & ([TModel] extends [AbstractControl] ? object : [TModel] extends [Array<infer U>] ? ReadonlyArrayLike<MaybeFieldTree<U, number>> : TModel extends Record<string, any> ? Subfields<TModel> : object);
 
 // @public
 export type FieldValidator<TValue, TPathKind extends PathKind = PathKind.Root> = LogicFn<TValue, ValidationResult<ValidationError.WithoutField>, TPathKind>;

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

@@ -6,10 +6,10 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
+import ts from 'typescript';
 import {runInEachFileSystem} from '../../src/ngtsc/file_system/testing';
 import {loadStandardTestFiles} from '../../src/ngtsc/testing';
 import {NgtscTestEnvironment} from './env';
-import ts from 'typescript';
 
 const testFiles = loadStandardTestFiles({forms: true});
 
@@ -44,7 +44,7 @@ runInEachFileSystem(() => {
       const diags = env.driveDiagnostics();
       expect(diags.length).toBe(1);
       expect(extractMessage(diags[0])).toBe(
-        `Type 'null' is not assignable to type '() => FieldState<string, string | number>'.`,
+        `Type 'null' is not assignable to type 'FieldTree<string, string | number>'.`,
       );
     });
 
@@ -100,7 +100,7 @@ runInEachFileSystem(() => {
       const diags = env.driveDiagnostics();
       expect(diags.length).toBe(1);
       expect(extractMessage(diags[0])).toBe(
-        `Type '() => FieldState<number, string | number>' is not assignable to type '() => FieldState<string, string | number>'.`,
+        `Type 'FieldTree<number, string | number>' is not assignable to type 'FieldTree<string, string | number>'.`,
       );
     });
 
@@ -124,7 +124,7 @@ runInEachFileSystem(() => {
       const diags = env.driveDiagnostics();
       expect(diags.length).toBe(1);
       expect(extractMessage(diags[0])).toBe(
-        `Type '() => FieldState<unknown, string | number>' is not assignable to type '() => FieldState<string | number | Date | null, string | number>'.`,
+        `Type 'FieldTree<unknown, string | number>' is not assignable to type 'FieldTree<string | number | Date | null, string | number>'.`,
       );
     });
 
@@ -187,7 +187,7 @@ runInEachFileSystem(() => {
       const diags = env.driveDiagnostics();
       expect(diags.length).toBe(1);
       expect(extractMessage(diags[0])).toBe(
-        `Type '() => FieldState<string, string | number>' is not assignable to type '() => FieldState<boolean, string | number>'.`,
+        `Type 'FieldTree<string, string | number>' is not assignable to type 'FieldTree<boolean, string | number>'.`,
       );
     });
 

packages/forms/signals/src/api/types.ts

@@ -168,12 +168,12 @@ export type FieldTree<TModel, TKey extends string | number = string | number> =
     : FieldState<TModel, TKey>) &
     // Children:
     ([TModel] extends [AbstractControl]
-      ? unknown
+      ? object
       : [TModel] extends [Array<infer U>]
         ? ReadonlyArrayLike<MaybeFieldTree<U, number>>
         : TModel extends Record<string, any>
           ? Subfields<TModel>
-          : unknown);
+          : object);
 
 /**
  * The sub-fields that a user can navigate to from a `FieldTree<TModel>`.