unbounded type is not strict

[royling]

TypeScript Version: 4.0.2

Search Terms: unbounded

Code

type Strings = [string, string];
type Numbers = number[];

type Unbounded = [...Strings, ...Numbers, boolean];
//   ^?

const data: Unbounded = ['a', 'b', false, false]; // <-- No error?
console.log(data);

Expected behavior:
The above Unbounded type should allow only the last element in the tuple as boolean, other elements in range [2, -2] if available should be numbers only.

Actual behavior:
[...number[], boolean] is now handled as <number | boolean>[]

Playground Link:
https://www.typescriptlang.org/play?#code/C4TwDgpgBAysBOBLAdgcwM5QLxQNroRVQBooCk0BdAbgChRIoA5AVwFsAjCeTHZdrvFw1a9cNACqyDgHsWyACYQF2PADoNcChlIa1rTt3SlZMgDYQAhshEB6W1EcA9APyiAxjOQEoCy8EsALigpWXklFRxcAHJLaNJojnioADNLM3QIUjSMiBFPb3MINTMZVAAKPwCASmogA

Related Issues:
No.

[RyanCavanaugh]

Tuple types don't support this operation, but we should be issuing the same error at the declaration site as we would if there wasn't a type alias used here

Shorter

type Numbers = number[];
type Unbounded = [...Numbers, boolean];
const data: Unbounded = [false, false];

[royling]

Tuple types don't support this operation

Ryan Cavanaugh (@RyanCavanaugh) This seems opposite of the update in 4.0 doc:

The second change is that rest elements can occur anywhere in a tuple - not just at the end!

[RyanCavanaugh]

Previously, rest elements could only occur at the end. Now, they can occur anywhere, as long as they meet the criteria of either being bounded-length tuples themselves, or being type parameters.

[royling]

Thanks for the reply.

as long as they meet the criteria of either being bounded-length tuples themselves, or being type parameters.

Should this be mentioned in the doc? Or, the Unbounded example used in Variadic Tuple Type looks inappropriate, IMHO.