infer function generic signature from actual function's return type

Suggestion

function test<T>(fn: (prev: T) => T) { }
test((prev) => ({ a: 1 })); // T is inferred as "unknown"

🔍 Search Terms

Maybe an issue exists, but I wasn’t sure what to search for. “Infer generic function type from return value” didn’t really help.

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code. not sure
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

⭐ Suggestion

T should be inferred as the type of the returned object, {a: number}

📃 Motivating Example

https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABFApgZygHgCoD4AUwYAXIvgA4BOKAbqdgJSIC8uijiA3ogL4BQqDPgrUaTVmW4BDUgEZeDBgG4+QA

💻 Use Cases

make life easier

[RyanCavanaugh]

I believe what actually happens is that we collect candidates for T, find out there are none, so default it to its constraint, which is unknown, and then process the call as if you had written test<unknown>(.

It’s weird since if you had written test<unknown>(, that definitely shouldn’t be an implicit any error. Maybe we need to make a marker unknown to use in zero-candidate inference that isn’t allowed to contextually type a parameter - worth experimenting with, probably.

[RyanCavanaugh]

@trusktr did you mean to use prev in these examples? Again we have the problem of “you can just omit the parameter”.