fix: relax form typings for union types (#15687)

`someField.set(...)` now works on union types; it no longer only accepts
the intersection of all unions.

Similarly, `someField.someSubProperty` now works on unions, though at
the slight cost of potentially accessing a combination of things you
shouldn't be able to. But I think that's better than the other
workarounds you have to do, and you can always narrow it if you want to
(in a much easier way than the other way around).

Fixes #14667

Author: dummdidumm

.changeset/solid-seas-know.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: relax form typings for union types

packages/kit/src/exports/public.d.ts

@@ -1932,6 +1932,15 @@ type RemoteFormFieldMethods<T> = {
 	issues(): RemoteFormIssue[] | undefined;
 };
 
+// These two types use "T extends unknown ? .. : .." to distribute over unions.
+// Example: if "type T = A | b" then "keyof T" only contains keys that both A and B have, with "KeysOfUnion<T>" we get the keys of both A and B
+type KeysOfUnion<T> = T extends unknown ? keyof T : never;
+type ValueOfUnionKey<T, K extends PropertyKey> = T extends unknown
+	? K extends keyof T
+		? T[K]
+		: never
+	: never;
+
 export type RemoteFormFieldValue = string | string[] | number | boolean | File | File[];
 
 type AsArgs<Type extends keyof InputTypeMap, Value> = Type extends 'checkbox'
@@ -2004,14 +2013,15 @@ export type RemoteFormFields<T> =
 		? RecursiveFormFields
 		: NonNullable<T> extends string | number | boolean | File
 			? RemoteFormField<NonNullable<T>>
-			: T extends string[] | File[]
+			: // [T] is used to prevent distributing over union, only the last condition should distribute over unions
+				[T] extends [string[] | File[]]
 				? RemoteFormField<T> & { [K in number]: RemoteFormField<T[number]> }
-				: T extends Array<infer U>
+				: [T] extends [Array<infer U>]
 					? RemoteFormFieldContainer<T> & {
 							[K in number]: RemoteFormFields<U>;
 						}
 					: RemoteFormFieldContainer<T> & {
-							[K in keyof T]-?: RemoteFormFields<T[K]>;
+							[K in KeysOfUnion<T>]-?: RemoteFormFields<ValueOfUnionKey<T, K>>;
 						};
 
 // By breaking this out into its own type, we avoid the TS recursion depth limit

packages/kit/test/types/remote.test.ts

@@ -2,6 +2,7 @@ import { query, prerender, command, form, requested } from '$app/server';
 import { StandardSchemaV1 } from '@standard-schema/spec';
 import {
 	RemoteForm,
+	RemoteFormFields,
 	RemoteFormInput,
 	RemotePrerenderFunction,
 	RemoteQueryFunction,
@@ -442,6 +443,49 @@ function form_tests() {
 		form.fields.allIssues();
 	}
 	void f10;
+
+	const f11 = form(
+		null as any as StandardSchemaV1<{
+			differing: { type: 'a'; propA: string } | { type: 'b'; propB?: string };
+		}>,
+		(data) => {
+			data.differing.type === 'a' || data.differing.type === 'b';
+			if (data.differing.type === 'a') {
+				data.differing.propA === '';
+				// @ts-expect-error
+				data.differing.propB;
+			} else {
+				data.differing.propB === '';
+				// @ts-expect-error
+				data.differing.propA;
+			}
+			return { success: true };
+		}
+	);
+	f11.fields.differing.issues();
+	f11.fields.differing.value().type === 'a' || f11.fields.differing.value().type === 'b';
+	const f11_field = f11.fields.differing.value();
+	if (f11_field.type === 'a') {
+		f11_field.propA === '';
+		// @ts-expect-error
+		f11_field.propB;
+	}
+	f11.fields.differing.propA.value();
+	f11.fields.differing.propB.issues();
+	// @ts-expect-error
+	f11.fields.differing.propC.value();
+	f11.fields.differing.set({ type: 'a', propA: 'test' });
+	f11.fields.differing.set({ type: 'b', propB: 'test' });
+	f11.fields.differing.set({ type: 'b' });
+	// @ts-expect-error
+	f11.fields.differing.set({ type: 'b', propA: 'test' });
+	// @ts-expect-error
+	f11.fields.differing.set({ type: 'a', propB: 'test' });
+	// test that you can narrow yourself if you want to
+	const f11_field2 = f11.fields.differing as RemoteFormFields<{ type: 'a'; propA: string }>;
+	f11_field2.propA;
+	// @ts-expect-error
+	f11_field2.propB;
 }
 form_tests();
 

packages/kit/types/index.d.ts

@@ -1906,6 +1906,15 @@ declare module '@sveltejs/kit' {
 		issues(): RemoteFormIssue[] | undefined;
 	};
 
+	// These two types use "T extends unknown ? .. : .." to distribute over unions.
+	// Example: if "type T = A | b" then "keyof T" only contains keys that both A and B have, with "KeysOfUnion<T>" we get the keys of both A and B
+	type KeysOfUnion<T> = T extends unknown ? keyof T : never;
+	type ValueOfUnionKey<T, K extends PropertyKey> = T extends unknown
+		? K extends keyof T
+			? T[K]
+			: never
+		: never;
+
 	export type RemoteFormFieldValue = string | string[] | number | boolean | File | File[];
 
 	type AsArgs<Type extends keyof InputTypeMap, Value> = Type extends 'checkbox'
@@ -1978,14 +1987,15 @@ declare module '@sveltejs/kit' {
 			? RecursiveFormFields
 			: NonNullable<T> extends string | number | boolean | File
 				? RemoteFormField<NonNullable<T>>
-				: T extends string[] | File[]
+				: // [T] is used to prevent distributing over union, only the last condition should distribute over unions
+					[T] extends [string[] | File[]]
 					? RemoteFormField<T> & { [K in number]: RemoteFormField<T[number]> }
-					: T extends Array<infer U>
+					: [T] extends [Array<infer U>]
 						? RemoteFormFieldContainer<T> & {
 								[K in number]: RemoteFormFields<U>;
 							}
 						: RemoteFormFieldContainer<T> & {
-								[K in keyof T]-?: RemoteFormFields<T[K]>;
+								[K in KeysOfUnion<T>]-?: RemoteFormFields<ValueOfUnionKey<T, K>>;
 							};
 
 	// By breaking this out into its own type, we avoid the TS recursion depth limit