types: add U of field<T, U>() to pass the preferred type as the result of toObject()

Author: mascii

docs/api/field.md

@@ -3,10 +3,14 @@ This function takes an initial value and a yup schema.
 This function returns `Field` object.
 
 ## Parameters
-The `field()` takes 3 parameters:
+The `field()` takes 3 parameters and 2 generic type parameters:
 
 ```typescript
-function field<T>(value: T | Ref<T>, schema?: FieldSchema, validateOptions?: ValidateOptions): Field<T>
+function field<T, U extends T = T>(
+  value: T | Ref<T>,
+  schema?: FieldSchema,
+  validateOptions?: ValidateOptions
+): Field<T, T extends U ? T : U>;
 ```
 
 ### 1. `value` <Badge type="danger" text="Required" />
@@ -67,6 +71,12 @@ Since `validateSync()` is used internally, Async tests are not available.
 ### 3. `validateOptions` <Badge type="info" text="Optional" />
 - Pass `{ abortEarly: false }` if all validation errors by the yup schema are required.
 
+### `T` <Badge type="info" text="Optional" />
+- Pass the type of `$value` explicitly like `ref<T>()`.
+
+### `U` <Badge type="info" text="Optional / Default: T" />
+- Pass the preferred type as the result of [`toObject()`](/api/toObject).
+
 ## Details of `Field` object
 ### `$value` property
 - The field value that can be set to `v-model`.

src/index.ts

@@ -32,7 +32,7 @@ export type FormPropType<T extends () => Form> = PropType<
   ReturnType<T> & { $key?: number }
 >;
 
-class Field<T> {
+class Field<T, U extends T = T> {
   private readonly $_valueRef: Ref<T>;
   private readonly $_errorRef: ComputedRef<yup.ValidationError | undefined>;
   public readonly $label: string;
@@ -210,11 +210,11 @@ export function defineForm<T extends Form>(form: T): T {
   return form;
 }
 
-export function field<T>(
+export function field<T, U extends T = T>(
   value: T | Ref<T>,
   schema?: FieldSchema,
   validateOptions?: ValidateOptions
-): Field<T> {
+): Field<T, T extends U ? T : U> {
   return new Field(value, schema, validateOptions);
 }
 
@@ -268,8 +268,8 @@ type ToObjectOutput<T extends Form> = {
     ? never
     : Exclude<K, "$key">]: T[K] extends Form
     ? ToObjectOutput<T[K]>
-    : T[K] extends Field<any>
-    ? T[K]["$value"]
+    : T[K] extends Field<any, infer U>
+    ? U
     : T[K] extends FormsField<(arg: any) => Form>
     ? ToObjectOutput<T[K]["$forms"][number]>[]
     : never;