typeof generic function with a specific template type should be possible

[brandon942]

Suggestion

⭐ Suggestion

function func<T>(a: T) { return a }
var value : ReturnType<typeof func<number>> // should be possible

Related to #37181 but even simpler - have a concrete type instead of a template type.
I wonder how this simple thing is not possible.

💻 Use Cases

function getValue<T>() {
	var a : T
	return { a: a }
}
var value
value = getValue<number>() // assigned later lazily
value // any (no typehint)

Currently it is impossible to have a variable be of a specific type that involves a generic function.

var value : ReturnType<typeof getValue<number>> // Error!

One has to create extra functions as a workaround, just to get a typehint for a variable:

// A workaround
function getValue<T>() {
	var a : T
	return { a: a }
}
function getValue_number() { // Extra function workaround just to avoid template syntax in typeof!
	return getValue<number>()
}
var value : ReturnType<typeof getValue_number>
// ...
value = getValue<number>() // assigned later
value // { a: number } - OK

🔍 Search Terms

typeof generic function

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• 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.

[RyanCavanaugh]

Duplicate #40542 ? I'm not sure what distinguishes this, #40542, and #37181 - they all seem to be the same thing IMO

[brandon942]

I'm not sure what distinguishes this, #40542, and #37181

Probably the expected operator precedence of typeof.
This opened issue focuses on the simpler case for which there is even a workaround. Defining a concrete type from a referenced generic function.
I don't see a workaround for the template type version.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.