[TS2.5 regression] generic type argument derivation with ": this" types

[mprobst]

TypeScript Version: Version 2.7.0-dev.20180110

Code

interface MsgConstructor<T extends Message> {
  new(data: Array<{}>): T;
}
class Message {
  clone(): this {
    return this;
  }
}
interface MessageList<T extends Message> extends Message {
  methodOnMessageList(): T[];
}

function fetchMsg<V extends Message>(protoCtor: MsgConstructor<V>): V {
  return null!;
}

class DataProvider<T extends Message, U extends MessageList<T>> {
  constructor(
      private readonly message: MsgConstructor<T>,
      private readonly messageList: MsgConstructor<U>,
  ) {}

  fetch() {
    const messageList = fetchMsg(this.messageList);
    messageList.methodOnMessageList();
  }
}

Expected behavior:

Compiles, because V is derived to be a subtype of MessageList. This is what it did in TS 2.5.

Actual behavior:

With TS2.6, derivation works differently:

repro.ts(25,17): error TS2339: Property 'methodOnMessageList' does not exist on type 'Message'.

If you explicitly pass fetchMsg<U>(...), you can see why:

repro.ts(24,34): error TS2344: Type 'U' does not satisfy the constraint 'Message'.
  Type 'MessageList<T>' is not assignable to type 'Message'.
    Types of property 'clone' are incompatible.
      Type '() => MessageList<T>' is not assignable to type '() => U'.
        Type 'MessageList<T>' is not assignable to type 'U'.

This seems wrong - MessageList inherits from Message, so any MessageList should satisfy the constraint Message.