Suggestion: use receiver's declared type arguments as defaults to its class constructor

未指定文件、测试或编译器入口点。首先查看 GenericClass 和 Map 示例，然后跟踪如何根据接收表达式推断构造函数类型参数。只有明确处理上下文默认值、间接调用、派生类和构造函数语义后，才算完成。

I run into this relatively simple pattern a lot:

class GenericClass<T, U, V> {
    a: T;
    b: U;
    c: V;
}

let x: GenericClass<number, string, boolean>;

x = new GenericClass();
// Which produces the error:
// Error: GenericClass<{}, {}, {}> is not assignable to GenericClass<number, string, boolean>

Since x has already been defined with a particular set of type arguments, these arguments could be implicitly assumed as defaults to its type's constructor when invoked with it being the receiver (for simplicity I'm assuming here it is the exact constructor as declared e.g. not a derived class).

I understand there will be cases where that would be very difficult or even impossible to achieve:

let createInstance = () => new GenericClass()
x = createInstance();

but these are relatively rare. Most of the time it's just:

class Example {
    myMap: Map<string, Array<{n: number}>>;

    constructor() {
        this.myMap = new Map<string, Array<{n: number}>>(); // <-- Why repeat?
    }
}

Which instead could have been:

class Example {
    myMap: Map<string, Array<{n: number}>>;

    constructor() {
        this.myMap = new Map(); // <-- Better.. :)
    }
}

• Any possible issues with this? perhaps ones that I'm not aware of or haven't thought of?
• Potential name collisions? (though I'm not aware of any?)
• Maybe just a bit too difficult to implement? perhaps revisit later? (though #5256 appears more difficult than this and seems to be in consideration?)
• Or maybe this 'bends' the semantics of the constructor expression (new Class<T>()), or at least its default type arguments, in a way that's slightly unappealing from a "purist" point of view"? (though this doesn't seem like a "purist" language?). Perhaps some concept of "contextual baseline default type arguments" can be included as an integral part of the future proposal for default type arguments?