Type Confusion when overwriting field on object creation

Bug Report

🔎 Search Terms

• "type confusion" -union label:Bug
• object field -union label:Bug

... too many Bugs in this tracker to to more than skim the titles

🕗 Version & Regression Information

See in 4.9.5 after running into an example for this in a project.

The Playground also reproduces it in 3.3.3 and the 5.0.0 nightly.

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

⏯ Playground Link

[Playground link with relevant code](https://www.typescriptlang.org/play?ts=4.9.5#code/C4TwDgpgBAGlC8UDeUCGAuKBnYAnAlgHYDmANFAEaaECuAthRLlAL4BQbAxgPaE5SdcEVMGiIAFADN8EADYATAHKo6ETAGsIIbpNjlCEAO4BBavUa4AlJjjwAfMjZQoQ4DVyFkaTAHJUP8iooACZyAG1pOSUVCABdaiNjVjZ2Ll5+AA8EASERCHE-AJDLNL5uWQgAOlluYnFQSB0oDMrUSygAeg6oWgYmNiA

)

💻 Code

type X = { a: string, b: number }

const create = (fieldName: keyof X, newA: number): X => {
  return { a: 'a', b: 2, [fieldName]: newA }
}

const x = create('a', 2)
console.log(typeof x.a) // number

🙁 Actual behavior

The code survives typechecks and has a number in x.a.

🙂 Expected behavior

TypeScript rejecting code that assigns a number to a field typed as string.

Das Issue stellt eine Reproduktion im TypeScript Playground bereit, nennt aber keine Datei oder keinen Test im Repository. Beginne damit, das Beispiel auszuführen und nachzuverfolgen, wie der Compiler die berechnete Eigenschaft im Objektliteral prüft; die Aufgabe ist erledigt, wenn TypeScript die dem String-Feld zugewiesene Zahl ablehnt.