打字稿中的“扩展"实际上是什么意思?

在联合类型的情况下，我无法完全掌握 extends 关键字的使用.这是一段代码片段，解释了我的困惑.

I am not able to fully grasp the use of extends keyword in case of union types. Here is a code snippet explaining the confusion that I have.

class SomeClass {
    someClassProp: string;
};

class SomeExtendedClass extends SomeClass {
    someExtendedClassProp: string;
};

function someClassFunction<T extends SomeClass>(args: T): T {
    return args;
};

let someClass: SomeClass, someExtendedClass: SomeExtendedClass;

someClassFunction(someClass);
someClassFunction(someExtendedClass); // Works just fine(case 1)

type someType = 'a' | 'b';
type someExtendedType = someType | 'c';

function someTypeFunction<T extends someType>(args: T): T {
    return args;
};

let someType: someType, someExtendedType: someExtendedType;

someTypeFunction(someType);
someTypeFunction(someExtendedType); // Gives me an error(case 2)

所以我想知道为什么要做出这样的设计决定以及相同的含义是什么.

So I was wondering why such design decisions are made and what are the implications of the same.

改变function someTypeFunction(args: T): T {return args;};到function someTypeFunction(args: T): T {return args;};工作，但我无法理解这件事实际上是如何工作的.

changing function someTypeFunction<T extends someType>(args: T): T {return args;}; to function someTypeFunction<T extends someExtendedType>(args: T): T {return args;}; works but I am not able to understand how this thing is actually working.

编辑 1

type someType = 'a' | 'b';
type someOtherType = 'c';
type someUnionType = someType | someOtherType;

type someTypeCore<T extends someUnionType> = { type: T };

type someTypeObj<T extends someType> = { a: string } & someTypeCore<T>;
type someOtherTypeObj<T extends someOtherType> = { b: string, c: string } & someTypeCore<T>;

function typeAssertion<T extends someUnionType>(args: someTypeCore<T>): args is someTypeObj<T> {
    return (args as someTypeObj<T>).a !== undefined; // Gives an error
};

function someTypeFunction<T extends someUnionType>(args: someTypeCore<T>): T {
    if (typeAssertion(args)) {
        // Do something for someTypeObj 
    } else {
        // Do Something for someOtherTypeObj 
    };
    return args.type;
};

我们如何解决这个问题.

How do we resolve this.