Object access reports error after upgrading to TypeScript 5.5

🔎 Search Terms

"noUncheckedIndexedAccess", "Object is possibly 'undefined'."

🕗 Version & Regression Information

• This changed between versions 5.4.5 and 5.5.3

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&ts=5.5.3#code/DYUwLgBAzmBOBc04EsB2BzCAfCBXVAJiAGZogEDcAUMsRABQCyAhmABYB0szhA9gLb0AlBAA8EAAwcAjCIDeVCElgQAvBABEzDVQC+VKgGNeqGBGaHDiORADaqZvxCIYsNOgC6iVLn4AjEBVdNQg5XWpaBlcIAEJ1H2BgeUVzS1tXDwgAfiz1CWolC0N0uA8AajLqfSA

💻 Code

// @strict: true
let str: string | undefined;
if (Math.random() < 0.1) {
  str = "a"
}

const acc: { [name: string]: number } = {};
if (str != null) {
  acc[str] ??= 0;
  acc[str]++; // In TS 5.5: Object is possibly 'undefined'. (2532)
}

🙁 Actual behavior

In TS 5.5.x, with noUncheckedIndexedAccess enabled, acc[str]++; reports an error "Object is possibly 'undefined'. (2532)".

🙂 Expected behavior

In TS 5.4.x and before, no errors were reported.

Additional information about the issue

I admittedly didn't do a very deep dive on the root cause here, but wanted to file an issue because intuitively it felt unexpected when upgrading to TS 5.5 that the above code now fails type-checking. Apologies if I missed an existing issue for this or something in the release notes!

Start with the linked TypeScript Playground repro and compare its diagnostics under 5.4.5 and 5.5.3, focusing on noUncheckedIndexedAccess, nullish assignment, and indexed access. Trace the checker behavior responsible for the regression and add a regression test; done means the example no longer reports the erroneous diagnostic while the relevant safety checks remain intact.