[5.0] Intersected array of intersection types with union field not compatible

[intgr]

Bug Report

Basically I'm using & types as if subclassing. BaseValue has items array with fewer properties, ExtendedValue extends that with items with more properties.

🔎 Search Terms

intersection, ampersand, override, inheritance. Also looked at the titles of first 2 pages of open issues.

🕗 Version & Regression Information

• This changed between versions 4.9.5 and 5.0.2
• There is an easy work-around, but I'm not sure if it should be necessary.

⏯ Playground Link

TypeScript 5.0.2 (error) vs 4.9.5 (OK)

💻 Code

type BaseItem = {
 id: number;
}
type ExtendedItem = BaseItem & {
 description: string | null
};


type BaseValue = {
 // there are other fields
 items: BaseItem[];
}
type ExtendedValue = BaseValue & {
 // there are other fields
 items: ExtendedItem[];
}


const TEST_VALUE: ExtendedValue = {
 items: [ // ❌ ERROR: This is NOT allowed *iff* mixed types are used in `description`
 {id: 1, description: null},
 {id: 2, description: 'wigglytubble'},
 ]
};

🙁 Actual behavior

Seems to me like they should be compatible. Also these examples are ✅ allowed in TS 5.0.2 if description only has uniform types, e.g.:

// ✅ Allowed: description has only strings
const ONLY_STRINGS: ExtendedValue = {
 items: [
 {id: 1, description: 'bollywoggle'},
 {id: 2, description: 'wigglytubble'},
 ]
};
// ✅ Allowed: description has only nulls
const ONLY_NULLS: ExtendedValue = {
 items: [
 {id: 1, description: null},
 {id: 2, description: null},
 ]
};

🙂 Expected behavior

Seems like it should be allowed.

🙄 Work-around

Typing ExtendedValue with Omit<> seems to fix the issue with no obvious downsides.

type ExtendedValue = Omit<BaseValue, 'items'> & {
 // there are other fields
 items: ExtendedItem[];
}

[typescript-bot]

The change between origin/release-4.9 and origin/release-5.0 occurred at 3099385.

[RyanCavanaugh]

#52392

[ahejlsberg]

This issue looks to be caused by incomplete propagation of the intersectionState parameter in our type relation logic. I have a fix that I'll put up shortly.