Inconsistent type narrowing behavior between compound assignment operators and their expanded equivalents

[hanwenguo]

🔎 Search Terms

"assignment operator" "type narrowing"

🕗 Version & Regression Information

• This is the behavior in every version in the Playground that I tried, and I reviewed the FAQ for entries about assignment operators and narrowing (there isn't any, though)

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241116#code/GYVwdgxgLglg9mABAdzgJwNYGcAUAHAQzQIFsAuRLKNGMAc0QB9EwQSAjAUzQEoKqa9RAG8AUIkSFiJRAF5JRUogDUiAEQE1AbnGI0nKCDRIppLYgD0FxAEEIETniicAJhQDkpku8QwsLIjQ4ZFdEKDhEdwFaOndRAF9RUVBIWARKAAs4EAAbFxscrDgAdXQMACEQKAAROE4sADk4KHxFckpqGKYWNi5efk6hMQkvFXkNbV19Q2MFaXMrRAAVAE88TkQAUTQgtA8vHz9EAH0wZuOAneDQ8Mjo+jj4oA

💻 Code

function works(param: string | number): string {
  param = param + "a";
  return param; // Accepted: 'param' is narrowed to 'string'
}

function shouldAlsoWorkButDoesNot(param: string | number): string {
  param += "a";
  return param; // Type Error: 'param' is _not_ narrowed to 'string'
}

🙁 Actual behavior

Compound assignments (+=) and their expanded equivalents (x = x + ...) behave differently: param = param + "a" correctly narrows but param += "a" does not.

🙂 Expected behavior

+= should behave equivalently to x = x + ... with respect to type narrowing.

Additional information about the issue

The code example is taken from here.

Another less relevant issue which also reveals the inconsistency between += and x = x + ... is #60521.

[jcalz]

duplicate-ish of #9815 and/or #17204 and/or #44553? Also related to #47027 (as linked above).