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Unexpected Error for conditional type parameter if value is generic typed #50066
Description
Bug Report
🔎 Search Terms
generic
conditional type
error
🕗 Version & Regression Information
Happens in every version available in TS Playground, including Nightly.
This is a crashThis changed between versions ______ and _______- This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics and conditional type
I was unable to test this on prior versions because _______
⏯ Playground Link
Playground link with relevant code
💻 Code
const doSomething : <T>(
input : (T extends undefined ? never : T) | null
) => void = (input) => {
console.log(input);
}
/* The unexpected Error */
const functionWithGenericType = <T extends string | null>(input : T) => {
doSomething(input); // Error: Type 'string' is not assignable to type 'T extends undefined ? never : T'.
}
/* Just some test to proove that the rest of my code is working as expected… */
const greeting : string | null = Math.random() > 0.5 ? 'Hello' : null;
doSomething(greeting); // No Error. As expected.🙁 Actual behavior
An error appears when calling doSomething(input) from inside a function with generic typed input.
🙂 Expected behavior
No Error appears.
Activity
MartinJohns commentedon Jul 27, 2022
Resolving of conditional types involving unbound generic type arguments is deferred. At that point the compiler doesn't know what
T extends undefined ? never : Tresolves to, because the compiler doesn't know what typeTis. Constraints are not taken into consideration. Same as #48243.When actually calling the function the type argument is not unbound anymore, so the conditional type can be resolved.
typescript-bot commentedon Jul 30, 2022
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.