Use declared type parameters in signature help

[mjbvz]

From: microsoft/vscode#25363

TypeScript Version: 2.3.1

Code

type Z = { abc: number, xyz: string }

function foo<T>(z: keyof T) { }

foo<Z>('')

Trigger suggestions inside of the quotes in the function call.

Expected behavior:
Returns suggestions for abc and xyz

Actual behavior:
No suggestions returned in this case.

If a non-generic type is used:

type Z = { abc: number, xyz: string }

function foo(z: keyof Z) { }

foo('')

Then suggestions are returned

[ghost]

The real problem is that we don't use a declared type parameter when getting signature help.

declare function f<T>(x: T): void;
f<number>(

Signature help here still shows f<T>(x: T): void.

[dkirchhof]

I don't know, if it fits exactly to this problem, but i'm struggling with the autocompletion while using keyof.
Small example:

function keyofTest<Type>(context: Type, key: keyof Type)
{
	return context;
}

class Test
{
	a: string;
	b: number;
}

keyofTest<Test>(new Test(), "a") // this works as expected
keyofTest(new Test(), "") // this not really

The second function call shows me all possible types, while the cursor is before the comma, but not between the quotes anymore. A wrong key would also be marked. Two images attached to see the behaviour:

[ghost]

@dkirchhof In the second case type inference has failed, since we can't infer any type Type such that new Test() : Type and "" : keyof Type. All parameters are used for type inference, not just the first. In this case you really only need the first, but it would be hard to change type inference to know about that, so instead it infers from all parameters and fails.

[dkirchhof]

Sorry, but i don't get it. At first, it shows me all possible keys (key: "a" | "b"), then it can't infer the type anymore and if a type in "c" or something else, which isn't a key of Test, it will be marked as a wrong argument until i change it to "a" or "b". So it just don't work for the "suggestions popup"?!

[ghost]

It works in the compiler layer but that information doesn't get to services which provides the completions. I've made an issue #20843.