New 'unknown' top type

[ahejlsberg]

This PR adds a new top type unknown which is the type-safe counterpart of any. Anything is assignable to unknown, but unknown isn't assignable to anything but itself and any without a type assertion or a control flow based narrowing. Likewise, no operations are permitted on an unknown without first asserting or narrowing to a more specific type.

Note that this PR is technically a breaking change since unknown becomes a reserved type name.

// In an intersection everything absorbs unknown

type T00 = unknown & null;  // null
type T01 = unknown & undefined;  // undefined
type T02 = unknown & null & undefined;  // null & undefined (which becomes never in union)
type T03 = unknown & string;  // string
type T04 = unknown & string[];  // string[]
type T05 = unknown & unknown;  // unknown
type T06 = unknown & any;  // any

// In a union an unknown absorbs everything

type T10 = unknown | null;  // unknown
type T11 = unknown | undefined;  // unknown
type T12 = unknown | null | undefined;  // unknown
type T13 = unknown | string;  // unknown
type T14 = unknown | string[];  // unknown
type T15 = unknown | unknown;  // unknown
type T16 = unknown | any;  // any

// Type variable and unknown in union and intersection

type T20<T> = T & {};  // T & {}
type T21<T> = T | {};  // T | {}
type T22<T> = T & unknown;  // T
type T23<T> = T | unknown;  // unknown

// unknown in conditional types

type T30<T> = unknown extends T ? true : false;  // Deferred
type T31<T> = T extends unknown ? true : false;  // Deferred (so it distributes)
type T32<T> = never extends T ? true : false;  // true
type T33<T> = T extends never ? true : false;  // Deferred

// keyof unknown

type T40 = keyof any;  // string | number | symbol
type T41 = keyof unknown;  // never

// Only equality operators are allowed with unknown

function f10(x: unknown) {
    x == 5;
    x !== 10;
    x >= 0;  // Error
    x + 1;  // Error
    x * 2;  // Error
    -x;  // Error
    +x;  // Error
}

// No property accesses, element accesses, or function calls

function f11(x: unknown) {
    x.foo;  // Error
    x[5];  // Error
    x();  // Error
    new x();  // Error
}

// typeof, instanceof, and user defined type predicates

declare function isFunction(x: unknown): x is Function;

function f20(x: unknown) {
    if (typeof x === "string" || typeof x === "number") {
        x;  // string | number
    }
    if (x instanceof Error) {
        x;  // Error
    }
    if (isFunction(x)) {
        x;  // Function
    }
}

// Homomorphic mapped type over unknown

type T50<T> = { [P in keyof T]: number };
type T51 = T50<any>;  // { [x: string]: number }
type T52 = T50<unknown>;  // {}

// Anything is assignable to unknown

function f21<T>(pAny: any, pNever: never, pT: T) {
    let x: unknown;
    x = 123;
    x = "hello";
    x = [1, 2, 3];
    x = new Error();
    x = x;
    x = pAny;
    x = pNever;
    x = pT;
}

// unknown assignable only to itself and any

function f22(x: unknown) {
    let v1: any = x;
    let v2: unknown = x;
    let v3: object = x;  // Error
    let v4: string = x;  // Error
    let v5: string[] = x;  // Error
    let v6: {} = x;  // Error
    let v7: {} | null | undefined = x;  // Error
}

// Type parameter 'T extends unknown' not related to object

function f23<T extends unknown>(x: T) {
    let y: object = x;  // Error
}

// Anything but primitive assignable to { [x: string]: unknown }

function f24(x: { [x: string]: unknown }) {
    x = {};
    x = { a: 5 };
    x = [1, 2, 3];
    x = 123;  // Error
}

// Locals of type unknown always considered initialized

function f25() {
    let x: unknown;
    let y = x;
}

// Spread of unknown causes result to be unknown

function f26(x: {}, y: unknown, z: any) {
    let o1 = { a: 42, ...x };  // { a: number }
    let o2 = { a: 42, ...x, ...y };  // unknown
    let o3 = { a: 42, ...x, ...y, ...z };  // any
}

// Functions with unknown return type don't need return expressions

function f27(): unknown {
}

// Rest type cannot be created from unknown

function f28(x: unknown) {
    let { ...a } = x;  // Error
}

// Class properties of type unknown don't need definite assignment

class C1 {
    a: string;  // Error
    b: unknown;
    c: any;
}

Fixes #10715.

[RyanCavanaugh]

Where'd we end up with mapped conditional types w.r.t unknown ?

[weswigham]

type T41 = keyof unknown; // string | number | symbol

Does that make sense (is this derived from something else)? If unknown is effectively the infinite union - a union of all possible types would have no common keys among all of them, leading me to believe the correct result is never (the given types seem correct for never, this being a place where any is more never-like than top-like).

[ahejlsberg]

@weswigham No, that probably doesn't make sense. We should be similar to keyof {} and keyof object here, both of which are never.

[ahejlsberg]

@RyanCavanaugh Do you mean the behavior of unknown in a distributive conditional type? It's an atomic type so it doesn't distribute. This is unlike {} | null | undefined which distributes and therefore is very hard to test for.

[weswigham]

I think a function returning an explicit unknown should probably have return values? Otherwise you should've written void or undefined, right?

[ahejlsberg]

Not sure about that. undefined is in the domain of unknown (just like it is in the domain of any) and that's really what should guide us here.

[weswigham]

Hm. Fair. Do we handle unions with undefined in accordance with that, then?

[ahejlsberg]

We do, but with a few wrinkles. If the return type annotation includes void, any, or unknown we don't require any return statements. Otherwise, if the return type includes undefined we require at least one return statement somewhere, but don't require return statements to have expressions and allow the end point of the function to be reachable. Otherwise, we require a return statement with an expression at every exit point.

[mhegazy]

why did this type collapse to true?

[ahejlsberg]

I changed this to eagerly resolve to true when the extends type is any or unknown (since it always will be).

[weswigham]

Ah, but what about T extends any ? { x: T } : never - that needs to distribute.

[ahejlsberg]

Hmm, yes, I suppose we can only optimize when the conditional type isn't distributive.

[weswigham]

There's a better check (than just distributivity) that I have an outstanding PR for: instantiate the check type (and infer types) in the true/false types with wildcards. If the instantiation isn't the input type, you can't simplify. If it is, you can. (Since the type isn't affected by instantiation)