Since 4.8.2, union discrimination fails with `(string & {})` when `noUncheckedIndexedAccess` is `true`

[maxijonson]

Bug Report

🔎 Search Terms

noUncheckedIndexedAccess
string & {}
union
intersection reduction

🕗 Version & Regression Information

4.8.2

• This changed between versions 4.7 and 4.8
• Possibly related to this PR, where (string & {}) should be an exception
• Only when noUncheckedIndexedAccess: true

⏯ Playground Link

Playground link with relevant code

💻 Code

// TS 4.8.2
// This does not fail by default under strict mode, since noUncheckedIndexedAccess is false by default
// Switch noUncheckedIndexedAccess to true to see the error
// Then switch to 4.7.4 to see the error disappear

type Alignment = (string & {}) | "left" | "center" | "right";
type Alignments = Record<Alignment, string>;

const a: Alignments = {
    left: "align-left",
    center: "align-center",
    right: "align-right",
    other: "align-other",
};

console.log(a.left.length); // This shouldn't be considered as possibly "undefined", since "left" is a defined Alignment (incorrect behaviour under 4.8.2)
console.log(a.other.length); // This should be considered as possibly "undefined" (correct behaviour)

tsconfig.json

{
  "compilerOptions": {
    "target": "ESNext",
    "module": "ESNext",
    "strict": true,
    "noUncheckedIndexedAccess": true
  }
}

🙁 Actual behavior

TypeScript 4.8.2 reports that an object's property is possibly "undefined", even for declared fields in the union.

🙂 Expected behavior

TypeScript 4.8.2 should only report that an object's property is possibly "undefined" for un-declared fields, as stated in the documentation

[fatcerberus]

This seems to be the relevant difference:

// 4.7.4 hover
type Alignments = {
    [x: string & {}]: string;
    left: string;
    center: string;
    right: string;
}

// 4.8.2 hover
type Alignments = {
    [x: string]: string;
}

Alignment itself doesn't reduce to string - the difference is in the behavior of Record, which isn't covered at all in #49119.

[maxijonson]

This seems to be the relevant difference:

// 4.7.4 hover
type Alignments = {
    [x: string & {}]: string;
    left: string;
    center: string;
    right: string;
}

// 4.8.2 hover
type Alignments = {
    [x: string]: string;
}

Alignment itself doesn't reduce to string - the difference is in the behavior of Record, which isn't covered at all in #49119.

Thanks for narrowing down the inferred type! You're right, Record isn't covered in that PR. It's worth noting that this issue is relevant for non-Record too. The same expected behavior (4.7.4) and actual behavior (4.8.2) happens here:

type Alignment = (string & {}) | "left" | "center" | "right";
type Alignments = {
  [key in Alignment]: string;
};

const a: Alignments = {
  left: "align-left",
  center: "align-center",
  right: "align-right",
  other: "align-other",
};

console.log(a.left.length); // Fine under 4.7.4. Error under 4.8.2
console.log(a.other.length); // Error for both versions (expected, however, in 4.8.2, I don't believe it's failing for the 'right reason')

[lll000111]

Not only unions. Here is a simpler example:

https://www.typescriptlang.org/play?ts=4.8.2#code/GYVwdgxgLglg9mABAZQKZgCYCEA2cBGAFBgIZQkBci4A1mHAO5gA0i6EcGMYA5lbfSYBKRAG8AsAChEMxDGCJC7Ttx6IAhAF5N1TKmDdUGRADITbSCt4btiAOQgowABx2RE6bK9QAFgCdGRDBUBkQAUT8Av0I7ZS5eNwBuKS8AXykU2QB6LMQAWTgQAGdURB84ADdUP0QoOEQS0oxUKFRoI1qATwAHUsyZHMQAdR90aiLVRAAVZEQKgBYAOmdFgCY5KDkisVTEAB9dZoNgjH7EQZGx4smZuaWAdkX5ja3EACJHFzf9w-1DU88MjiqmSklSoKAA

function SendBlob(data: unknown, encoding: unknown) {
    if (encoding !== undefined && encoding !== 'utf8') {
        throw new Error('encoding');
    }

    // Mouse hover to see detected type 
    // When using TS v4.8.2 it is {} | undefined
    // When using TS v4.7.4 it is "utf8" | undefined
    encoding;
};

[fatcerberus]

That’s… not even close to being the same problem as described here.

[maxijonson]

While this is not in fact an issue relating to (string && {}) in declared union types or noUncheckedIndexAccess, I believe it could be related, since the conditions for encoding could create an "inferred" union.

[lll000111]

For the record, definitely related, now its own ticket: #50706