Type narrowing does not apply to default arguments in definition time

Bug Report

The default values for function parameters are evaluated during definition, and late-bound defaults aren't a thing yet, so type narrowing should work for them just fine.

To Reproduce

# demo.py
from typing import Optional

foo: Optional[int] = 42
assert foo is not None
reveal_type(foo)  # demo.py:5: note: Revealed type is "builtins.int"

def bar(arg: int = foo) -> None:  # demo.py:7: error: Incompatible default for argument "arg" (default has type "Optional[int]", argument has type "int")
    ...

Expected Behavior

There should be no error.

Actual Behavior

demo.py:7: error: Incompatible default for argument "arg" (default has type "Optional[int]", argument has type "int")

Your Environment

• Mypy version used: 0.961 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.10.1
• Operating system and version: Ubuntu 20.04

Start with the reproduction in demo.py and run it with the reported Mypy setup to confirm that narrowing is lost for the default argument. Trace the handling of narrowed expressions used in function defaults, then add a regression test showing that the Optional[int] value narrowed by the assertion is accepted as an int default.