Literal slicing of a variable-length tuple does not work

Bug Report

Slicing a variable-length tuple with a literal int does not reveal as a corresponding fixed-length tuple. This causes both false negative, where no error reported with incorrect unpacking, and false positive of incompatible type with say return values.

To Reproduce

def foo(x: tuple[str, ...]) -> tuple[str, str]:
    a, b = x[:3]

    return x[:2]

Expected Behavior

a, b = x[:3] should give an error of too many values to unpack, return x[:2] should pass with no error.

Actual Behavior

a, b = x[:3] gives no error, return x[:2] says incompatible return value type.

Your Environment

• Mypy version used: 0.961
• Python version used: 3.10
• Operating system and version: macOS 12.4

Start by reproducing the provided tuple-slicing example with mypy 0.961 on Python 3.10 and inspect the type-checking path for literal slices of variable-length tuples. Done means reporting an error for unpacking x[:3] into two values while accepting x[:2] as tuple[str, str].