fix: widen remote form field types

[elliott-with-the-longest-name-on-github]

closes #16931

Union fields

Given a schema like { a: string } | { b: string }, form.fields was typed as { a: RemoteFormField<string> } | { b: RemoteFormField<string> }. That meant neither form.fields.a nor form.fields.b could be accessed, even though both fields exist on the runtime proxy:

const f = form(schema, () => {});

f.fields.a.as('text'); // error
f.fields.b.as('text'); // error

This preserves the original input union in a tuple so the outer conditional does not distribute over it, then deliberately collects the keys from every union member for the form instance. The container methods still use the original union, while the instance exposes both a and b; unknown fields remain errors.

Enum fields

A string enum field kept its literal union in the attributes returned by .as(...), which made an otherwise valid spread fail type checking:

<input {...form.fields.choice.as('text', 'one')} />

This widens literal strings to string only in the returned DOM-facing value property. The .as(...) arguments still use the original enum, so choice.as('text', 'three') remains an error for a 'one' | 'two' field.

Both cases have focused type regression tests.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 8e0ee5e:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/8e0ee5ec6ff170476c515959c78bbad43f9c1958

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16937

[changeset-bot]

🦋 Changeset detected

Latest commit: 8e0ee5e

The changes in this PR will be included in the next version bump.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16937-svelte.vercel.app/