narrowing of property should also narrow its parent when optional chaining

[DetachHead]

Suggestion

🔍 Search Terms

narrowing optional chaining

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

when narrowing a property in an object, there are often other things it should be able to narrow as well. for example, when narrowing foo?.bar to a non-undefined type, it should also narrow foo to not be undefined

📃 Motivating Example

declare const foo: {
  bar: number[]
} | undefined

if (Array.isArray(foo?.bar)) {
  foo.bar //error: Object is possibly 'undefined'
}

💻 Use Cases

i'm trying to conditionally modify an array, but only if its parent property is present. currently, i have to use ! to tell the compiler that it's not undefined, which isn't ideal:

if (
  itemDefinition.request === undefined ||
  typeof itemDefinition.request.url === 'string'
)
  return itemDefinition
const newItemDefinition = cloneDeep(itemDefinition)
if (Array.isArray(newItemDefinition.request!.url)) {
  newItemDefinition.request!.url.push({})
}

[MartinJohns]

This sounds like a duplicate of #34974 to me.