Unbounded arrays cannot be used as rest elements in a tuple type except at the end

Bug Report

🔎 Search Terms

A rest element must be last in a tuple type.

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about rest elements and tuples.

Specifically, I tried it with 4.1.5 right now. Note that 4.0.5 behaved a bit differently (but still buggy), see below.

Note that the behavior is normal in versions ≤4.0, since that’s when non-final rest elements were allowed in tuples.

⏯ Playground Link

Playground link with relevant code

💻 Code

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

type Unbounded = [...Strings, ...Numbers, boolean]; 

🙁 Actual behavior

The ...Numbers part generates an error: TS1256: A rest element must be last in a tuple type.

This exact code appears in TypeScript 4.0 release notes, thus presumably should work. (Search for “the resulting type becomes unbounded”.)

Note: I encountered something similar with 4.0.5 earlier. There, the non-final unbounded rest elements worked, but only via a type alias. They didn’t work with literal arrays in the tuple definition:

type Numbers = number[];

type Unbounded1 = [boolean, ...number[]]; // OK
type Unbounded2 = [...Numbers, boolean]; // OK
type Unbounded3 = [...number[], boolean]; // NOK

Playground Link for TS 4.0.5

🙂 Expected behavior

The feature should work as documented. TypeScript should generate unbounded tuples type that work the “obvious” way.

Start with the linked TypeScript Playground repro and the tuple type example showing an unbounded rest element before a boolean. Trace the compiler's rest-element validation for tuple types, then confirm that the documented TypeScript 4.0 example is accepted and that the resulting unbounded tuple behaves as expected.