Intellisense missing for generic type extending an all partial type

[AviVahl]

TypeScript Version:
3.1.3 and 3.2.0-dev.20181110

Search Terms:
Intellisense suggestions generic type

Code

interface Options {
    someFunction?: () => string
    anotherFunction?: () => string
}

export class Clazz<T extends Options> {
    constructor(public a: T) {
    }
}

new Clazz({

// ctrl + space on above line does not provide any suggestion
// it does when:
// - making some/anotherFunction required instead of optional (in Options)
//   OR
// - giving constructor's first param a type of `T | Options`
})

Expected behavior:
suggest someFunction and anotherFunction.

Actual behavior:
no suggestions

Playground Link:
http://www.typescriptlang.org/play/#src=interface%20Options%20%7B%0D%0A%20%20%20%20someFunction%3F%3A%20()%20%3D%3E%20string%0D%0A%20%20%20%20anotherFunction%3F%3A%20()%20%3D%3E%20string%0D%0A%7D%0D%0A%0D%0Aexport%20class%20Clazz%3CT%20extends%20Options%3E%20%7B%0D%0A%20%20%20%20constructor(public%20a%3A%20T)%20%7B%0D%0A%20%20%20%20%7D%0D%0A%7D%0D%0A%0D%0Anew%20Clazz(%7B%0D%0A%0D%0A%2F%2F%20ctrl%20%2B%20space%20in%20line%2012%20does%20not%20provide%20any%20suggestion%0D%0A%2F%2F%20it%20does%20when%3A%0D%0A%2F%2F%20-%20making%20some%2FanotherFunction%20required%20instead%20of%20optional%20(in%20Options)%0D%0A%2F%2F%20%20%20OR%0D%0A%2F%2F%20-%20giving%20constructor's%20first%20param%20a%20type%20of%20%60T%20%7C%20Options%60%0D%0A%7D)%0D%0A

Related Issues:

[parzh]

Looks like I also encountered this issue, so +1. Hope, this playground is useful:

type Direction = "up" | "down" | "left" | "right";

type MovementDescriptor<Dir extends Direction> = {
    [D in Dir]: true;
};

// NB: The error here is that the `MovementDescriptor` type
// does not suggest type parameter autocompletion on Ctrl + Space
// (to test that, change type parameter to "" and press Ctrl + Space with cursor inside the parens),
// while all the compile errors occur just fine as expected
// (to test that, try making an typo, e.g. changing "left" to "let").
interface MovementUp extends MovementDescriptor<"up" | "left"> { }

const up: MovementUp = {
    down: true, // compile error is expected
}; 

[cspotcode]

I'm hitting this as well. Here is our use-case:

Our codebase builds large JSON API responses from multiple partial objects that we splat together. I'd like to have Intellisense for property names; we use verbose and inconsistent property names. But I'd also like some validation that, when the partial objects are splatted together, the result includes all properties. We don't want to accidentally omit anything.

So I need some way to:
a) get tab-completion for property names
b) let us omit properties when creating these partial objects
c) infer which properties have been specified so we can prove that the merged object contains all necessary properties

Naively, I tried inferring the type of each partial object via function partialApiResponse<T extends Partial<ApiReponseJson>(t: T) {return t} But Intellisense doesn't offer any suggestions.

[cspotcode]

I noticed another behavior that suggests this is an unintended bug rather than an intentional limitation.

playground link

interface Foo {
    a_a: boolean;
    a_b: boolean;
    a_c: boolean;
    b_a: boolean;
}
function partialFoo<T extends Partial<Foo>>(t: T) {return t}
partialFoo({
    // <-- Ctrl-Space shows empty completion list.  Typing additional letters continues to show empty list.
    // *BUT* if you type the letter "a" *first*, you see a completion list, which then narrows as you type.
});

EDIT: Someone on reddit gave me a workaround: infer the keys union and type the argument as Partial<> & Pick<>:

partialFoo<T extends keyof Foo>(part: Pick<Foo, T> & Partial<Foo>): Pick<Foo, T> {return t;}

[HaloWang]

Come from #29180 with VSCode version 1.31.1. This problem still exist.😫

[HaloWang]

#306

[maxfontana90]

I found a similar issue with a generic interface. It doesn't infer prop names even if its explicitly declared. Is this ok?

[parzh]

@maxfontana90 In your case it is fine, since by stating that someImmutableGenericObject is a GenericObject you remove all the information about its properties, that might have otherwise be inferred. Try removing : Readonly<GenericObject>, this might bring IntelliSense back.

Also, technically this object is neither immutable (you can mutate its state), nor a Generic. See the meaning of the latter term here.

[KillyMXI]

I think I have an issue related to this.

interface IBase {
  /** Base property */
  readonly a: number
}

interface ISpecialX extends IBase {
  /** Property X */
  readonly x: number
}

interface ISpecialY extends IBase {
  /** Property Y */
  readonly y: number
}

type SomethingSpecial = ISpecialX | ISpecialY;

export function doSomething(input: SomethingSpecial): void {}

And then:

import { doSomething } from '...';

doSomething({ a: 1, x: 2 });

Here, TypeScript language server can report a wrongly formed object passed as an argument (omit property a - get a type mismatch report).
But IntelliSense tooltip only shows documentation for properties from special interfaces, namely property x in this case. For the property a it shows only information inferred from usage.