if MYPY guarding an always-return block v. unreachable

Bug Report

Given a block dependent on if MYPY, if it always returns, mypy will conclude that everything following is unreachable. This shouldn't be obviously different to using an else block.

To Reproduce

from typing import Any, cast

MYPY = False

def foo(p):
    # type: (Any) -> int
    doc = p
    if MYPY:
        return cast(int, doc)
    else:
        # needs to be in else for MYPY to believe this is reachable
        return doc

def bar(p):
    # type: (Any) -> int
    doc = p
    if MYPY:
        return cast(int, doc)
    return doc

Expected Behavior

Either both output [unreachable] or both type check successfully.

Actual Behavior

foo.py:19: error: Statement is unreachable  [unreachable]

i.e.: only bar fails to typecheck

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.0
• Operating system and version: Arch Linux

No source file or test is named. First reproduce the example with mypy 0.910 and --warn-unreachable, then trace the reachability handling for an if MYPY block versus an else block. Done means both equivalent examples either type-check successfully or report unreachable consistently.