Unsafe Map overload spoils direct property access on parenthesized assignment expression

[Andarist]

🔎 Search Terms

map overload any unknown unsafe weakmap set binary assignment lazy init

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250326#code/DYUwLgBAxghlAWIBcECyMAOAeAzmATgJYB2A5gDQQD2ARgFYhRgB8EAPhAK7EAmIAZiRA8A3ACgx-bk0JViEMCDwBGABQBrEAE8UeImQCUEAN5iI0OXgj4lnYJAC8EVbAQgIAfg9PiIAO5omKoGBgB0pOAa2gbi5gD0cebmAHoeZtbgnPjyNjh2YOIAvhJSxDJyCkpgAExROhB6JKRGpuauiJ7eEL4B6BjBsRbEVrn5EE7tIOGRmlox6QlJEKnpNmBZObb2RWJAA

💻 Code

let cache: Map<string, object> | undefined;

function test1(key: string) {
  const result = (cache ??= new Map()).get(key);
  //    ^? const result: any
  return result;
}

function test2(key: string) {
  cache ??= new Map();
  const result = cache.get(key);
  //    ^? const result: object | undefined
  return result;
}

🙁 Actual behavior

First result gets spoiled as any

🙂 Expected behavior

I'd expect both results to be object | undefined

Additional information about the issue

This bug reports an issue related to Map but, tbf, it's not unique to Map and a more general solution would have to be found to properly fix this. The problem is how checkBinaryLikeExpressionWorker create a union of left and right types here - without taking into consideration that the assignment target might already overlap with the right type. That union type gets reduced with UnionReduction.Subtype and .get gets read from it - instead of the assigned to reference on the left side.

[nmain]

I've run into this before but never reported it because I assumed it was unfixable without changing new () => Map<any, any>, IE, #60051.

[Andarist]

@nmain this only uses Map as an example but any other custom function could suffer from this. IMHO, test2 shows that it's not impossible to fix this