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.

The issue provides a TypeScript Playground reproduction but names no repository file or test. Start by running the example and tracing how the compiler checks the computed property in the object literal; it is done when TypeScript rejects the number assigned to the string field.