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TypeScript can't infer type of default parameters #59643
Description
🔎 Search Terms
infer default parameters
function wrapper
function factory
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ
- I tried down to version 4.0.3 (since in the earlier version
Generatortype is not generic)
- I tried down to version 4.0.3 (since in the earlier version
⏯ Playground Link
💻 Code
declare function fnDerive<A extends unknown[], R>(
fn: (...args: A) => Generator<unknown, R>,
): unknown /** mocked return */;
fnDerive(function *(a = 0, b = '') {
console.log({
a,
// ^?
b
// ^?
});
});🙁 Actual behavior
The type of a and b is unknown.
🙂 Expected behavior
The type of a and b should be number and string
Additional information about the issue
I was working with a library gensync when I hit this issue, and I extracted the minimum reproduction out of it.
Note that if I don't use default parameters, typescript can infer the a and b types correctly:
declare function fnDerive<A extends unknown[], R>(
fn: (...args: A) => Generator<unknown, R>,
): unknown /** mocked return */;
fnDerive(function *(a?: number | undefined, b?: string | undefined) {
console.log({
a,
// ^?
b
// ^?
});
a ??= 0;
b ??= '';
console.log({
a,
// ^?
b
// ^?
});
});
Activity
Andarist commentedon Aug 15, 2024
This is not specific to generator functions. It's related to inferring type parameters from unannotated parameters with inferred types from initializers (TS playground):
It's also just related to how there is a contextual signature available here. This touched on what I improved here: #56506 . This PR didn't break it anyhow when it comes to the case presented here but it also didn't improve it.
So the question here is also - how both situations should be differentiated? There are situations in which the contextual parameter type should be preferred. Perhaps it could be done based on
checkMode & CheckMode.Inferential, or maybe based on the uninstantiated contextual signature when the parameter's type is a type variable.I'd love to experiment with this but I have a long list of things to experiment with so no promises 😅
[-]TypeScript can't infer type of default parameters in generator functions[/-][+]TypeScript can't infer type of default parameters in factory functions[/+][-]TypeScript can't infer type of default parameters in factory functions[/-][+]TypeScript can't infer type of default parameters[/+]SukkaW commentedon Aug 15, 2024
Updated title and search terms to reflect this~