infer recursive type (Typescript)

[mjbvz]

From @MikkelSnitker on April 16, 2018 17:25

Hi.
I'm not sure if this is a bug/feature in vscode or in typescript, but recursive types resolves to any?
Giving the code below TestType3 resolves to:

TypeScript Type<{prop1: Type<String>,prop2: Type<any>}>
and not as i expected
TypeScript Type<{prop1: Type<String>,prop2: Type<{subprop: Type<String>}>

I'm not sure if this is a typescript issue, but the compiler catches the error if prop2 does not contain a string property with the name subprop.

type Constructor<T> = { new(...args: any[]): T }
type Type<T> = T & { validate?: (value:any)=>boolean};
type InferTest<T> = T extends Constructor<infer T1> ? Type<T1>: 
T extends Array<Constructor<infer T1>> ? Array<Type<T1>>:
Type<{[K in keyof T]: InferTest<T[K]> }> 

declare function Test1<T>(o: T): InferTest<T>;

const TestType1 = Test1(Number) // Type<Number>;
const TestType2 = Test1([Number]) // Type<Number>[];
const TestType3 = Test1({
    prop1: String,
    prop2: {
        subprop: String
    }
}) // Type<{prop1: Type<String>,prop2: Type<any>}> 

var test4: typeof TestType3 = {
    prop1: "Hello World!",
    prop2: { 
        subprop: 4 
    }
}

• VSCode Version: 1.22.1
• OS Version: 10.13.3

Does this issue occur when all extensions are disabled?: Yes

Regards Mikkel

Copied from original issue: microsoft/vscode#47984

[mhegazy]

The recursive type is inferred correctly. the issue is how it is printed when you hover over it.

First type aliases are not types, they are just aliases. in other words the compiler replaces every reference to them with the actually type.
Second when the compiler is serializing the type, and it runs in circularity it uses any to break write that.

[ahejlsberg]

Inference is actually yielding the expected type

Type<{ prop1: Type<String>, prop2: Type<{ subprop: Type<String> }> }>

The issue is simply that when displaying the type, the compiler only goes a couple of levels deep before substituting an any in order to avoid what might be an infinitely nested type. (In this case the type isn't infinitely nested, but we don't have a heuristic to detect that.)

[MikkelSnitker]

@mhegazy that was what i thought since i got a compile error. is there a way to rewrite (my code red) that so it displays the correct type when hovering?

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.