Mapped type property causses different assignability error of compatible subtypes

[dragomirtitian]

🔎 Search Terms

assignability mapped type sub type

🕗 Version & Regression Information

• This changed between versions 3.3 and 3.5, but the behavior was not correct in 3.3 either, 3.5 just makes it a bit better but less consistent.

⏯ Playground Link

Playground Link

💻 Code

interface Table<B> { schema: B; }

interface DerivedTable<S extends DerivedSchema<any, any>> extends Table<S["base"] & S["new"]>{
    // readonlySchema: Readonly<S["base"] & S["new"]>
}
type DerivedSchema<B, N> = { base: B; new: N };

interface Base { baseProp: number; }
interface New  { newProp: boolean; }

declare const source: DerivedTable<{ base: Base, new: New }>
const destination: DerivedTable<{ base: Base; new: New & Base }> = source;

🙁 Actual behavior

There is an error on the last line

🙂 Expected behavior

DerivedTable<{ base: Base, new: New }> and DerivedTable<{ base: Base; new: New & Base }> are both equivalent to Table<New & Base> and so the assignment on the last line should succeed. Commenting out the readonlySchema property will actually make the assignment succeed. At the very least this should always be an error as it was in 3.3.

Additional information about the issue

Found while testing tsgo which depending on type order might errors on it even with readonlySchema present. Reported as microsoft/typescript-go#1781

[RyanCavanaugh]

I don't think we're going to touch this in Strada given the age of the issue and likely unintended side effects. Let's track at the Corsa issue.

[typescript-bot]

This issue has been marked as "Won't Fix" and has seen no recent activity. It has been automatically closed for house-keeping purposes.