Add safer types of casts

The only form of cast expression we have is intended to completely bypass the type system. This is like most forms of casting in C (not counting conversions) -- it never fails either at runtime or at compile time.

Maybe we can add two new types of casts:

Downcast

downcast(T, E) checks at compile time that the type of expression E is a supertype of T, and checks at runtime that E is in fact an instance of T. As a special case, if the type of E is Any, the compile time check always succeeds, but the runtime check is still performed.

A possible implementation:

def downcast(type, expr):
    assert isinstance(expr, type)
    return expr

It's intentional that this uses assert (though debatable): the intended use case is currently handled by inserting the same assert manually. IOW:

x = downcast(type, expr)

is roughly equivalent to:

assert isinstance(expr, type)
x = expr

Upcast

Probably much less needed, but proposed for symmetry and because occasionally it's useful. upcast(T, E) should check at compile time that T is a supertype of the type of E, and at runtime it's a no-op.

A possible implementation:

def upcast(type, expr):
    return expr

This fragment:

x = upcast(type, expr)

is roughly equivalent to:

x: type = expr

except that it works even if the type of x has already been declared.

Bắt đầu bằng việc xem xét ngữ nghĩa downcast và upcast được đề xuất trong phần nội dung của issue. Không có tệp, bài kiểm thử hoặc entry point nào được nêu tên; bước đầu tiên là thống nhất một thiết kế, sau đó sẽ cần triển khai và viết các bài kiểm thử.