Type narrowing does not apply to default arguments in definition time

[PIG208]

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