Incorrect inferred function return type with destructuring assignments

提供された TypeScript Playground の repro から始め、ケース 1–4 を比較し、分割代入を介したジェネリックな戻り値型の推論に焦点を当てます。ケース 4 で結果が R に拡大される理由を追跡し、その後、他のケースは変更されないまま、A3.a で null または undefined のエラーが報告されなくなったことを確認します。

🔎 Search Terms

"return type destructure"
"destructuring assignment type"

🕗 Version & Regression Information

• This changed between versions 3.3.3 and 3.5.1

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.2#code/C4TwDgpgBASlC8UDeAoK6oEEBcy0YIENcByTE-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-Q9Y3tbMS-KwGpoiFd+ZtjI8hgdCVFKEwGv9sqhQeVobl4c22vCrLRcf1CaNSf+KYLGdLWZo9GYbA4XFeQhEExZueQgzTRZ5boF469vsjDblAFEQ-ZcAB5VIAK172MMYDk+n0AlSBt6U5YVNRfVn-Hn3F6QA

💻 Code

type R = {
    A: {
        a: 'A'
    } | null | undefined
}
function fn<T extends R>(t:T): T  {
    return {} as T;
}

/*
In each test case below, I'd assume the return type of function to be `{A: {a: 'A'}}`.
However, case 4 uses destructuring and this appears to default the return type to `R`.
*/

// Case 1
// Correctly infers type of `result.A` as `{A: {a: 'A'}}`
const result = fn({A: {a: 'A'}});
result.A.a; // No error

// Case 2
// Correctly infers type of A1 as `{a: 'A'}`
const A1 = fn({A: {a: 'A'}}).A;
A1.a; // No error

// Case 3
// Correctly infers type of A3 as `{a: 'A'}`
const {A: A2} = fn({A: {a: 'A'}} as const);
A2.a; // No error

// Case 4
// Incorrectly infers type of A2 as `{A: {a: 'A'} | null | undefined}`
const {A: A3} = fn({A: {a: 'A'}});
A3.a; // Error: Object is possibly `null` or `undefined`

🙁 Actual behavior

In case 4, accessing A3.a results in error "Object is possibly null or undefined".

Appears that the inferred type of A3 is {a: 'A'} | null | undefined. I suspect when we destructure, the return type of fn is inferred as R instead of the narrowed generic type variable T.

🙂 Expected behavior

Expect no errors. Should be able to access A3.a, similar cases 1, 2, and 3. Expectation is that destructuring assignment shouldn't modify inferred return type of function.

Additional information about the issue

Current workarounds:

• Use as const appended to param (see case 3)
• Use const with generic type variable in function signature (function fn<const T extends R>(t: T): T;)
• Avoid destructuring assignments (case 1 & 2)