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Type 'T' is not assignable to type 'T extends Sub ? Sub : T'. #48746
Description
Activity
Any recommendations on how to express this differently that doesn't run into this problem? Perhaps there is a way to express the join/meet operation without conditionals.
FWIW I tried using
[T] extends [Sub]to avoid distributive behavior but it doesn't help.Any recommendations on how to express this differently that doesn't run into this problem? Perhaps there is a way to express the join/meet operation without conditionals.
interface Base { base: unknown; } interface Sub extends Base { sub: unknown; } function transform<T extends Base>(t: T extends Sub ? T & Sub : T): T extends Sub ? Sub : T { return t; } var t = transform({base: 0, foo: 0}) // ^? var t: { base: number, foo: number } var sub = transform({base: 0, sub: 0, foo: 0}) // ^? var sub: Sub
I don't know why
T extends Sub ? T & Sub : Tis assignable toT extends Sub ? Sub : Tsince it doesn't meet the conditions mentioned in #46429.
Seems like a black magic 😎Maybe syntax-driven? Would make sense.
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on Oct 22, 2025
Bug Report
🔎 Search Terms
variance, conditional, subtype, limit
🕗 Version & Regression Information
⏯ Playground Link
Playground link with relevant code
💻 Code
🙁 Actual behavior
"Type 'T' is not assignable to type 'T extends Sub ? Sub : T'."
🙂 Expected behavior
No errors. Either T extends Sub, and so is assignable to it, or it doesn't, and so is assignable to itself.