Wrong "Too many arguments" errors with tuple unpacking

Bug Report

I'm getting wrong or misleading "Too many arguments" errors when using tuple unpacking in function calls.

To Reproduce

from typing import Tuple

def x(x: int, y: int) -> None:
    print(x, y)

def foo1(t: tuple) -> None:
    x(*t[:1], 1)

def foo2(t: tuple) -> None:
    x(*t[:2])

def foo3(t: Tuple[int]) -> None:
    x(*t[:1], 1)

mypy reports:

$ mypy sandbox.py 
sandbox.py:7: error: Too many arguments for "x"
Found 1 error in 1 file (checked 1 source file)

(Line 7 is the call to x in foo1)

Expected Behavior

The call to x in foo1 will never have too many arguments -- it has either 1 (if t is empty) or 2 (if t is not empty). I would have expected an error regarding the possibility of t being empty (i.e. "Potentially not enough arguments").

Also I think that foo1 and foo2 share the same issues regarding their calls of x, so I would have expected to get the same errors for both of them.

Actual Behavior

The call to x in foo1 is reported, but the call to x in foo2 isn't. Interestingly, fixing the tuple length (as in foo3) seems to fix the issue.

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.5
• Operating system and version: Ubuntu 18.04.5

Tái hiện các ví dụ trong sandbox.py bằng mypy trên môi trường Python 3.8.5 được báo cáo và so sánh các chẩn đoán cho foo1, foo2 và foo3. Truy vết việc kiểm tra đối số khi giải nén tuple bắt đầu từ lời gọi bị lỗi, sau đó thêm phạm vi kiểm thử hồi quy cho cách xử lý dự kiến của từng ví dụ và xác minh rằng các chẩn đoán nhất quán.