Argument of type 'this' is not assignable to parameter of type 'this'.

This error also appears on generics with this to the super call (which is the real use case).

class A {
    constructor(bar: this) { }
    foo(bar: this) { }
}

class B extends A {
    constructor() {
        super(this); // Error
        this.foo(this); // Works
    }
}

I'm not getting any errors on the latest bits from master
this is incorrect ES6 code since this is used before super call

Vladimir Matveev (@vladima)

Vladimir Matveev (@vladima) Don't know why no.2 is causing the same error but I did more test with the latest "next" release and found the actual issue:

class Test<TSample> {
    // this is the property triggering the issue.
    sample: TSample;

    constructor() { }
}

class Sample {
    constructor(
        public host: Test<this>
    ) { }
}

class TerminalSample extends Sample {
    constructor(host: Test<this>) {
        super(host);
    }
}

here is a simpler example:

class Base {
    constructor(a: this) { }
}

class Derived extends Base {
    prop: string;
}

var base: Base;
var derived: Derived;

new Base(base); // Error should be OK
new Derived(derived); // Error should be OK 
new Derived(base); // Error

The bug here is that the this type should not be in scope in the parameter list of a constructor (but it should be in scope within the constructor body). This is similar to how the this type is not in scope in static members.

Nathan Shively-Sanders (@sandersn)

Nathan Shively-Sanders (@sandersn) can you add the error check for release-1.7

There is an existing error for this, it's just getting missed now. I'll figure out why.

Nathan Shively-Sanders (@sandersn)

Nathan Shively-Sanders (@sandersn) The issue is in the getThisType method. When the container is a constructor we should be checking that the node is in the body of the constructor (and thus not in the parameter list).

Add tests based on #5449

So I just made my use case dead? I don't understand why we need to disallow this type in constructor parameter list.

vilic The use case was dead all along, we just weren't reporting an error properly. The compiler isn't equipped to handle this types in the static side of a class (which includes the constructor) and it would take a substantial amount of work to implement it. Is there a particular scenario where this would be useful?

Anders Hejlsberg (@ahejlsberg)

Anders Hejlsberg (@ahejlsberg) It's actually quite like what I wrote, more specifically:

abstract class Test<TSample> {
    sample: TSample;
}

class SomeTest extends Test<SomeSample> {

}

abstract class Sample {
    constructor(
        public host: Test<this>
    ) { }    
}

class SomeSample extends Sample {
    constructor(host: Test<this>) {
        super(host);
        host.sample = this;
    }
}

let someTest = new SomeTest<SomeSample>();
let someSample = new SomeSample(someTest);
someTest.host.sample; // expecting type `SomeSample`.

Actually I ended up with another implementation (as this is messing things up), but I think the scenario should be reasonable. I didn't realize that constructors have different contexts than other instance methods, but certainly it could wait long enough if it's not something trivial to do.

Add tests based on #5449

Navigation Menu

Repository navigation

Argument of type 'this' is not assignable to parameter of type 'this'. #5449

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Argument of type 'this' is not assignable to parameter of type 'this'. #5449

Description

Activity

vladima commented on Oct 29, 2015

vilicvane commented on Oct 29, 2015

mhegazy commented on Oct 29, 2015

ahejlsberg commented on Oct 30, 2015

mhegazy commented on Oct 30, 2015

sandersn commented on Oct 30, 2015

ahejlsberg commented on Oct 30, 2015

vilicvane commented on Oct 31, 2015

ahejlsberg commented on Oct 31, 2015

vilicvane commented on Oct 31, 2015

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions