fix: fix types

see microsoft/TypeScript#46855

Author: susisu

src/core/field.ts

@@ -1,5 +1,5 @@
 import { EventEmitter } from "./events";
-import { Disposable, ElementType } from "./shared";
+import { AsArray, Disposable, ElementType } from "./shared";
 
 export interface Field<T> extends EventEmitter {
   readonly id: string;
@@ -64,7 +64,9 @@ export interface ChildField {
 
 export interface FieldNode<T> extends Field<T> {
   createChild<K extends ChildKeyOf<T>>(key: K): ChildFieldNode<T[K]>;
-  createChildArray<K extends ChildArrayKeyOf<T>>(key: K): ChildFieldArray<ElementType<T[K]>>;
+  createChildArray<K extends ChildArrayKeyOf<T>>(
+    key: K
+  ): ChildFieldArray<ElementType<AsArray<T[K]>>>;
 }
 
 export interface ChildFieldNode<T> extends FieldNode<T>, ChildField {}

src/core/impl/fieldNode.ts

@@ -1,5 +1,5 @@
 import { ChildArrayKeyOf, ChildFieldArray, ChildFieldNode, ChildKeyOf } from "../field";
-import { ElementType } from "../shared";
+import { AsArray, ElementType } from "../shared";
 import { FieldImpl } from "./field";
 import { FieldArrayImpl } from "./fieldArray";
 import { Child, Getter, Parent, Setter } from "./shared";
@@ -56,7 +56,9 @@ export class FieldNodeImpl<T> extends FieldImpl<T> implements ChildFieldNode<T>
     return field;
   }
 
-  createChildArray<K extends ChildArrayKeyOf<T>>(key: K): ChildFieldArray<ElementType<T[K]>> {
+  createChildArray<K extends ChildArrayKeyOf<T>>(
+    key: K
+  ): ChildFieldArray<ElementType<AsArray<T[K]>>> {
     const path = `${this.path}.${String(key)}`;
     if (
       Object.getPrototypeOf(this.getDefaultValue()) !== Object.prototype ||
@@ -76,8 +78,8 @@ export class FieldNodeImpl<T> extends FieldImpl<T> implements ChildFieldNode<T>
         `You are creating a field array '${path}', but the value of '${path}' is not a pure array. This may cause unexpected errors.`
       );
     }
-    type E = ElementType<T[K]>;
-    const getter: Getter<T, readonly E[]> = value => value[key];
+    type E = ElementType<AsArray<T[K]>>;
+    const getter: Getter<T, readonly E[]> = value => value[key] as AsArray<T[K]>;
     const setter: Setter<T, readonly E[]> = (value, x) => {
       if (Object.is(value[key], x)) {
         return value;

src/core/index.ts

@@ -1,4 +1,4 @@
-export { Disposable, ElementType } from "./shared";
+export { AsArray, Disposable, ElementType } from "./shared";
 export {
   Field,
   Snapshot,

src/core/shared.ts

@@ -1,3 +1,5 @@
 export type Disposable = () => void;
 
 export type ElementType<T extends readonly unknown[]> = T[number];
+
+export type AsArray<T> = T extends readonly unknown[] ? T : never;

src/react/core.ts

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import {
+  AsArray,
   ChildArrayKeyOf,
   ChildKeyOf,
   CreateFormParams,
@@ -37,7 +38,7 @@ export function useChild<T, K extends ChildKeyOf<T>>(field: FieldNode<T>, key: K
 export function useChildArray<T, K extends ChildArrayKeyOf<T>>(
   field: FieldNode<T>,
   key: K
-): FieldArray<ElementType<T[K]>> {
+): FieldArray<ElementType<AsArray<T[K]>>> {
   const [childArray] = useState(() => field.createChildArray(key));
 
   useEffect(() => {