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.

Beginne mit der verlinkten TypeScript Playground-Reproduktion und dem Tupeltyp-Beispiel, das ein unbeschränktes Rest-Element vor einem boolean zeigt. Verfolge die Validierung von Rest-Elementen für Tupeltypen im Compiler und bestätige anschließend, dass das dokumentierte TypeScript 4.0-Beispiel akzeptiert wird und dass sich das daraus resultierende unbeschränkte Tupel wie erwartet verhält.