# Dom.IfEquals

Produces `Output` only when two types are mutually assignable.

## Signatures

```ts
export type IfEquals<X, Y, Output> = (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y ? 1 : 2 ? Output : never;
```

## Why

DOM host types use exact equality to identify writable properties safely.

## Ownership and lifetime

Type-only and resource-free.