Faulty declarations file from extending generic base class

[NoelAbrahams]

TypeScript Version: 2.4.1

Code

declare type Newable<TType> = {
    new (...params: any[]): TType;
};  

interface Bar<T>{
    (): T;
}

const FooBar = {};

class Foo<TItem> extends (FooBar as any as Newable<Bar<TItem>>) {

}

Expected behavior:
The generated declaration file should contain the following

declare type Newable<TType> = {
    new (...params: any[]): TType;
};
interface Bar<T> {
    (): T;
}
declare const FooBar: {};
declare const Foo_base: Newable<Bar<any>>;
declare class Foo<TItem> extends Foo_base {
}

Actual behavior:

declare type Newable<TType> = {
    new (...params: any[]): TType;
};
interface Bar<T> {
    (): T;
}
declare const FooBar: {};
declare const Foo_base: Newable<Bar<TItem>>;  // <----- TItem is not defined
declare class Foo<TItem> extends Foo_base {
}

When the generated declarations file is referenced by other code, the compilation fails with

error TS2304: Build:Cannot find name 'TItem'.

[weswigham]

@mhegazy The example in the OP is (correctly) an error nowadays: Base class expressions cannot reference class type parameters, thanks to #17922. If the OP is still active and curious why, see the discussion in #17829 which is somewhat similar to this.