Array prototype extensions fail circularly with `...args: T[]`, but not `args: T[]`

🔎 Search Terms

Typescript array prototype extension spread rest variadic parameter circular failure TS2310 version 5.4.3

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about circular compilation failures

I've added two playground links; the first works, the second fails. The only difference between the snippets is receiving an array T[] as a spread argument vs a single argument.

⏯ Playground Link

Working: https://www.typescriptlang.org/play?#code/PTAEBUAsEsGdQO4HsBOBrWAuAUAYyQHawAuoBApggLLnGRIAm8AvKAN7ahegCGDDAHnAA+ABR04mCAG0AugBpQANx4AbAK7ksM2QEop4dqAmwAdAAd1sSKNN2VGrboDcoFLXUoCytZtjSABllXAF9sEOdsYgBPc3JQADlKAEEUFB5omjpGFmNY8iQAMzJKLPomSOgCYnIUQp5ceNT06KFhUHIADxqCJkSUtIyynPYI7GwAeQAjACtyXGJTBnJCqvIABRQkOJQY0WaMiy3iJBi4xQByPgYLxTYOgnUAW1qeKdVyKXrVWHJFBBQ0GIbw+UmIKE0igcmikFGotHKZmuoBCLnG+CIpCe0WaUhIgIIAHM5KBWHJItjmqZrqJpKArlNcLd6ctCsyLoSYBdQHpInhCLAkB9TKokITRPdKWkUS4gA

Failing: https://www.typescriptlang.org/play?#code/PTAEBUAsEsGdQGYENoBtYC4BQBjA9gHawAuoBApgO4Cy5xkeAJvALygDeWo3oSjjAHnAA+ABT04GCAG0AugBpQAOhUA3JKgCu5TDNkBKKeA6gJsJQAdNsSKJVL1WnfoDcoAE51N7gqEfbYaQAGWTcAXywwlyxiAE8LclAAOSoAQXd3JFjaeiZWU3jyPAQyKhyGZmjoAmJyd2QcRPTM2KFhUHIAD1qCZmS0jKzyvI4orCwAeQAjACtyHGIlRnIEavIABXc8BPc40Wasyy3iPDiExQByPkYLxXYOgk0AWzqkKdRyKWR0ckVKd2gxDeHykxHc2kU-k+pRodAq5muoDCrnG+CIpCesWaUhIAIIAHM5KA2HJopjmkprqIrlMcLdQBdlgh6Rd8TALijcIRYHgPkpUHh8aJ7uSMkjXEA

💻 Code

Failing:

const newMethods = {
    add<T>(this: T[], ...values: T[]): T { this.push(...values); return values[0]; }
};
type NewArrayMethods = typeof newMethods;
interface Array<T> extends NewArrayMethods {};

Object.defineProperty(Array.prototype, 'add', { enumerable: false, writable: true, value: newMethods.add });

const myArr: string[] = [];
myArr.add('abc', 'def', 'ghi');

console.log({ myArr });

Working (only difference is ...values: T[] is now values: T[] - without ...):

const newMethods = {
    add<T>(this: T[], values: T[]): T { this.push(...values); return values[0]; }
};
type NewArrayMethods = typeof newMethods;
interface Array<T> extends NewArrayMethods {};

Object.defineProperty(Array.prototype, 'add', { enumerable: false, writable: true, value: newMethods.add });

const myArr: string[] = [];
myArr.add([ 'abc', 'def', 'ghi' ]);

console.log({ myArr });

🙁 Actual behavior

Example using ...values: T[] fails to compile!

🙂 Expected behavior

Both examples should compile successfully.

Additional information about the issue

No response

Start with the failing TypeScript Playground link and compare it against the working link, using the TypeScript 5.4.3 reproduction in the issue. Trace why the spread rest parameter produces TS2310 while the array parameter does not; done means both shown examples compile successfully without the circularity error.