exactOptionsPropertyTypes - anomolous behavior with Tuples, options, and spreading.

Bug Report

🔎 Search Terms

• strictOptionalProperties #43947 #44548
• exactOptionalPropertyTypes
• Labelled Tuple Elements
• Optional Tuple Parameters (enabled with labeling)
• Variadic Tuple Types

🕗 Version & Regression Information

• Spreading of Variadic Tuple types, as well as the ability to indicate final position optional parameters with labels, were introduced in ts 4.0. The noted behavior has been been present since then.

• However, although there was hope it would be fixed with 4.4's exact exactOptionsParameters, it was not.

⏯ Playground Link

https://www.typescriptlang.org/play?exactOptionalPropertyTypes=true&ts=4.4.2#code/ATAuE8AcFNgFQIzALzANoA8BcA7ArgLYBG0ATgDTAYD8WRA9vQDbQCGOAugFAgDG9OAM6gwCBFkQp0CcnhwATaADMAljmjyOwAPTbgAUQAaAYX0AZM-oBycAIS3gAdxWgAFsGgZWvUAHlIoCoCrEwACqT0MKQQcFDQgligpHiwbiqCwCS8rASw6cDsHqQRpAB0wDzA-EIioAgATBJIqGgySSncfALCogDMTVKt3JUQMPD1gwDkAHqT5KULiOSTACSTnVXdtfXicBMtM3NtydDLaxwA3JXVPaD1jXtTs+Qycoqq6vJn6xc6ejj0MikYAAWmAxjMAEEAJIAWVBYFc+QAygAJXwAVTMABFMrBCkD6KRrlswPV+o8Ds8ZKsfn8isCwRCYfCwWkMmjMTjgFZfHA8QUcAyiRUgA

💻 Code

  type T1 = [x:number, x?:boolean]
  const t11:T1 = [1,undefined] // EXCELLENT!! with exactOptionalPropertyTypes:true this became is an error. 
  const t12:T1 = [1,true]
  const t13:T1 = [1]

  type T2 = ['^',...T1,'$']
  const t21:T2 = ['^',1,true,'$'];
  const t22:T2 = ['^',1,undefined,'$']; // noerr - CLAIM - this SHOULD be an error
  const t23:T2 = ['^',1,'$']; // err - CLAIM - this SHOULD NOT be an error 

🙁 Actual behavior

In the above code, variable declaration for t22 is NOT an err.
In the above code, variable declaration for t23 IS an err.

🙂 Expected behavior

In the above code, variable declaration for t22 IS an err.
In the above code, variable declaration for t23 is NOT an err.

Even though with exactOptionalPropertyTypes , the T1 now no longer considers [number,undefined] to be a legal assignment, when T1 is spread into T2 the old behavior is used instead of the new behavior.

The documentation for exactOptionalPropertyTypes says

In TypeScript 4.4, the new flag --exactOptionalPropertyTypes specifies that optional property types should be interpreted exactly as written, meaning that | undefined is not added to the type:

exactOptionalPropertyTypes と T1/T2 のタプル代入を使用する、リンク先の TypeScript Playground の再現から始めてください。variadic tuples が展開されるときにオプショナルなタプル要素がどのように振る舞うかを調査し、その後、既存の T1 チェックが正しいままであることを確認しつつ、t22 が拒否され、t23 が受け入れられることを検証してください。