Immediately destructuring a spread union allows access to properties not present in all union constituents

[jcalz]

Bug Report

🔎 Search Terms

spread, destructuring, union

🕗 Version & Regression Information

• This changed between versions 3.5.1 and 3.6.2. Specifically it changed with Fix destructuring with fallback #31711. It is still present in 4.2.3 and in nightly.

⏯ Playground Link

Playground link

💻 Code

interface A { a: string }
interface B { b: number }

function foo(x: A | B) {
    const { a } = { ...x } // no error?! 
    // const a: string | undefined
    a?.toUpperCase();
}

const x = { a: 1, b: 2 }
foo(x); // a.toUpperCase is not a function, oops

🙁 Actual behavior

There is no error; a is inferred to be of type string | undefined.

🙂 Expected behavior

I expected an error on destructuring assignment to a saying Property 'a' does not exist on type 'A | B', as you would get with the seemingly equivalent construction without spreading

const { a } = x; // error! Property 'a' does not exist on type 'A | B'.

or the also-seemingly-equivalent construction with an intermediate variable

const v = { ...x }
const { a } = v; // error! Property 'a' does not exist on type '{ a: string; } | { b: number; }'

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I figure this is a side effect of #31711. But is it considered a bug? a design limitation? or working as intended? Comes from this SO question.