[1.20 regression] no longer accepts the empty tuple for `except ...:`

[TTsangSC]

Bug Report

mypy 1.20 no longer considers the empty tuple as a valid argument to except ...: unless it is explicitly annotated to be a (potentially empty) tuple of exception types. May be related to #17762 since the PR seems to be the last one that touched the exception-type-tuple handling for try-except block.

(Same context as #21149: the repo, the failed pipeline, and the caveat about mypy-version behaviors not lining up between mypy Playground and PyPI releases.)

To Reproduce

(Gist: https://gist.github.com/mypy-play/d0bd152e2e868b693ca73079096d13c6)

def this_no_longer_works() -> None:
    try:
        pass
    except ():
        assert False, 'This should be unreachable'
    

def but_this_still_does() -> None:
    accepted_errors: tuple[type[Exception], ...] = ()
    try:
        pass
    except accepted_errors:
        pass

(The real repo code of course doesn't just do a redundant try: ... except: ..., but uses it for e.g. optional post-mortem debugging.)

Expected Behavior

Type-check passes as with before

Actual Behavior

main.py:4: error: Exception type must be derived from BaseException (or be a tuple of exception classes)  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: master (on Playground)
• Mypy command-line flags: nil
• Mypy configuration options from mypy.ini (and other config files): nil
• Python version used: 3.14

[hauntsaninja]

Bisects to #17762