The info that generic type extends some type is lost during inheritance

[astashov]

TypeScript Version: 2.4.2

Code

class Value<T> {
  constructor(protected readonly data: T) {}

  public bar(data: Partial<T>): this {
    return this;
  }
}

interface Type {
  foo: string;
}

class TheClass<T extends Type> extends Value<T> {
  public foo(): this {
    return this.bar({foo: "d"});
  }
}

Expected behavior:

No errors, since it should be aware T extends Type, which has foo.

Actual behavior:

Shows an error:

Argument of type '{ foo: "d"; }' is not assignable to parameter of type 'Partial<T>'.

Playground

[DanielRosenwasser]

Seems related to #16985. Can you take a look @ahejlsberg?

[astashov]

I think it happened before 2.4 as well.

[DanielRosenwasser]

Actually this is technically correct behavior, not sure why I didn't catch that the first time.

[Jessidhia]

Why is this correct? Changing the class to class TheClass extends Value<Type> does correctly accept the Partial<Type> as an argument; <T extends Type> should have T be equivalent to Type for template type checking purposes, no?

[mhegazy]

{foo: "d"} is not assignable to Partial<T>, T is generic. see #19388 for an explanation.