Type guards don't infer generic types

[purplesyringa]

TypeScript Version: 4.0.0-dev.20200724

Search Terms: generics, type guard, typeof

Code

function func<T>(object: T): T {
	if(typeof object === "number") {
		return 1;
	} else {
		return object;
	}
}

Expected behavior:

The code compiles. It is obvious that if typeof object === "number", then T must be number, and 1 is a number.

Actual behavior:

The code yields:

test.ts:3:3 - error TS2322: Type '1' is not assignable to type 'T'.
  'T' could be instantiated with an arbitrary type which could be unrelated to '1'.

3  	return 1;
   	~~~~~~~~~


Found 1 error.

Playground Link: There we go

Related Issues: none

[RyanCavanaugh]

This is a correct error for the reason described in the error message.

// Causes the value '1' to inhabit an expression of type '2'
const a = func<2>(2);

[purplesyringa]

@RyanCavanaugh Perhaps that was a wrong example. My actual code is similar to:

function func<T>(object: T): T extends Number ? number : T {
	if(object instanceof Number) {
		return object.valueOf();
	} else {
		return object;
	}
}

I don't see how func<2>(2) or similar would apply here.

I'll reopen the issue and change the code in the first comment if this turns out to be a bug.

[RyanCavanaugh]

#33014 / #33912

[purplesyringa]

Sorry for duplicates. I'll try to search better next time.

[RyanCavanaugh]

Those ones are hard for me to find 😅