Polymorphic 'this' with Generics

[shlomiassaf]

TypeScript Version:
typescript@1.8.9
typescript@1.9.0-dev.20160404

Code

// 2 Simple interfaces, child extends parent.
interface ParentData {
    parentData: string;
}
interface ChildData extends ParentData {
    childData: string;
}


// 2 Simple classes with Generics.
// Each class has a "placeholder" method.
// Type parameter must satisfy the XData interface above.
class Parent<T extends ParentData> {
    toBeMethod: (value: string) => this;
}


// Here we have 2 "extends"
// 1) Type parameter T extends ChildData interface (which extends ParentData)
// 2) Child<T> extends Parent<T>, Parent will have a type parameter T that is ChildData (at least)
class Child<T extends ChildData> extends Parent<T> {
    yetAnotherToBeMethod: (value: string) => this;
}

// NOTE: The "placeholder" methods have a return signature of 'this', this + generics cause's the issue.

// These are factories for creating instances of Parent,Child
// 2 type parameters, T (with constraint) and Z also with constraint related to T.
export class ParentFactory<T extends ParentData, Z extends Parent<T>> {}
export class ChildFactory<T extends ChildData, Z extends Child<T>> extends ParentFactory<T, Z> {}

Expected behavior:
No errors...

Actual behavior:

error TS2344: Type 'Z' does not satisfy the constraint 'Parent<T>'.
  Type 'Child<T>' is not assignable to type 'Parent<T>'.
    Types of property 'toBeMethod' are incompatible.
      Type '(value: string) => Child<T>' is not assignable to type '(value: string) => Z'.
        Type 'Child<T>' is not assignable to type 'Z'.

Type 'Child' is not assignable to type 'Z'.

How Child is not assignable to type 'Z' if:
export class ChildFactory<T extends ChildData, Z extends Child> extends ParentFactory<T, Z> {}

[RyanCavanaugh]

Is this distinct from #6223?

[shlomiassaf]

An error isn't specified there, so it's hard to tell.
The compiled code works when I run it, I just get that error...

It's seems legit if I look at the constraints...

[shlomiassaf]

In #6223 it's referring to not suppling a generic argument vs suppling one... so I'm not sure but it might be related to the same underlying bug/not a feature.

[mhegazy]

i believe this is a bug. we are not instantiating the constraint with correct this argument before doing the comparison.

[shlomiassaf]

Here's a simpler sample that causes the error, with only 1 type parameter

// 2 Simple interfaces, child extends parent.
interface ParentData {
    parentData: string;
}
interface ChildData extends ParentData {
    childData: string;
}


// 2 Simple classes with Generics.
// Each class has a "placeholder" method.
// Type parameter must satisfy the XData interface above.
class Parent<T extends ParentData> {
    toBeMethod: (value: string) => this;
}


// Here we have 2 "extends"
// 1) Type parameter T extends ChildData interface (which extends ParentData)
// 2) Child<T> extends Parent<T>, Parent will have a type parameter T that is ChildData (at least)
class Child<T extends ChildData> extends Parent<T> {
    yetAnotherToBeMethod: (value: string) => this;
}

// NOTE: The "placeholder" methods have a return signature of 'this', this + generics cause's the issue.

// These are factories for creating instances of Parent,Child
export class ParentFactory<T extends Parent<ParentData>> {}
export class ChildFactory<T extends Child<ChildData>> extends ParentFactory<T> {}

Error:

error TS2344: Type 'T' does not satisfy the constraint 'Parent<ParentData>'.
  Type 'Child<ChildData>' is not assignable to type 'Parent<ParentData>'.
    Types of property 'toBeMethod' are incompatible.
      Type '(value: string) => Child<ChildData>' is not assignable to type '(value: string) => T'.
        Type 'Child<ChildData>' is not assignable to type 'T'.

[dsherret]

Here's another example:

abstract class Class1 {
    myMethod() {
        return this;
    }
}

class Class2 extends Class1 {
}

abstract class Test1<T extends Class1> {
}

class Test2<T extends Class2> extends Test1<T> { // error: type Class2 is not assignable to T
}

[mhegazy]

should be fixed by #7290

[dsherret]

This fixed the issue for me. Thank you!