Unsafe Map overload spoils direct property access on parenthesized assignment expression

🔎 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.

Reproduce la diferencia entre test1 y test2 usando el TypeScript Playground enlazado y, después, inspecciona checkBinaryLikeExpressionWorker, que el informe identifica como el encargado de formar la unión de los operandos de asignación. Sigue cómo se reduce esa unión antes de resolver .get. Se considera terminado cuando el acceso a la asignación entre paréntesis infiere object | undefined, al igual que la forma de asignación separada, con cobertura de regresión para el ejemplo indicado.