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Any TypeScript gurus in the house who can help me troubleshoot this? What am I fundamentally misunderstanding about type parameters / conditionals here?

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```ts
enum Type {
	A = 'A',
	B = 'B',
}

type FromThing<T extends Type = Type> = (
	& {
		type: Type,
	}

	& (
		T extends Type.A ?
			{
				A: number
			}
		
		: T extends Type.B ?
			{
				B: string
			}

		:
			Record<string, never>
	)
)

type ToThing<T extends Type = Type> = (
	& {
		type: Type,
	}

	& (
		T extends Type.A ?
			{
				a: number
			}
		
		: T extends Type.B ?
			{
				b: string
			}

		:
			Record<string, never>
	)
)

const transform = <
	T extends Type = Type
>(
	fromThing: FromThing<T>
): (
	ToThing<T>
) => ({
	type: fromThing.type,

	...(
		fromThing.type === Type.A && 'A' in fromThing && fromThing.A && {
			a: fromThing.A,
		}
	),

	...(
		fromThing.type === Type.B && 'B' in fromThing && fromThing.B && {
			b: fromThing.B,
		}
	),
})
```

Does it work if you use an `is` predicate with a symbol for "type"?

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