Function with this type can be assigned to a type without

[hesselink]

TypeScript Version: Version 2.8.0-dev.20180302

Search Terms: strict this assigned property

Code

The following code, compiled with --strict, gives a type error:

class C {
  f(this: this) : void {}
}
const c : C = new C();
const f = c.f;
f();

This correctly prints "error TS2684: The 'this' context of type 'void' is not assignable to method's 'this' of type 'C'."

However, I can assign a type to the variable f without the this context, and things will typecheck (but fail if the function actually uses this internally):

class C {
  f(this: this) : void {}
}
const c : C = new C();
const f : () => void = c.f;
f();

Expected behavior:

The code gives a type error like "Type '(this: C) => void' is not assignable to type '() => void'.

Actual behavior:

The code compiles without errors.

Playground Link: here

Related Issues:

[ericanderson]

Also fails on 2.7.2

[sebastiaanvisser]

Note that this issue has slightly broader consequences than the situation sketched above.

Because by default class methods don't auto-infer a this type on their signature you can pass them directly (unbound to this) as a callback to another function take the signature without this. This clearly causes runtime errors:

function twice(f: () => number) {
    alert(2 * f())
}

class C {
    constructor(private v: number) { }
    get() { return this.v }
}

const c: C = new C(2);

// Both typecheck:
twice(() => c.get()); // Will alert 4
twice(c.get); // Will alert NaN

[Conaclos]

The error is not reported even if the option "strict" is enabled.

declare const method: (this: Date) => void
declare let f: () => void
f = method // no error in TS 2.8-dev

[mattmccutchen]

Looks like a duplicate of #7968.

[hesselink]

@mattmccutchen That seems to be about a flag to change the default this type for functions, to provide better call-site and assignability checking. This is about the assignability checking itself not catching a type error, so I think this is a different issue.

[mattmccutchen]

@hesselink As #7968 suggests, the current behavior is that when the this type of a function signature is not specified, it defaults to any for the purpose of assignability checking. That's why the assignability check is not reporting an error in your example. So it's the same issue AFAICT.

[hesselink]

Ah you're right. I guess I'll close this ticket and subscribe to that one.