Error on conditional initialization of Final class variables

Bug Report

Conditional initialization of a Final class variable is reported as a reassignment error on elif/else branches.

To Reproduce

tmp.py:

from typing import Final, Optional


class Foo:
    a: Final[Optional[int]]

    def __init__(self, v: int) -> None:
        if v == 0:
            self.a = None
        else:
            self.a = v  # error: Cannot assign to final attribute "a"  [misc]

Expected Behavior

No errors on the first correct assignment of a final variable regardless of branching.

Actual Behavior

$ mypy tmp.py
tmp.py:11: error: Cannot assign to final attribute "a"  [misc]
Found 1 error in 1 file (checked 1 source file)

Reported error: Cannot assign to final attribute "a" [misc] during the else clause, even though it's only the first assignment of the variable.

Your Environment

• Mypy version used: 0.910

• Mypy command-line flags: mypy tmp.py

• Mypy configuration options from pyproject.toml (and other config files):

  [tool.mypy]
  show_error_codes = true
  

• Python version used: 3.9.5

• Operating system and version: macOS 11.4

Related

• Pyright has already fixed this issue: https://github.com/microsoft/pyright/issues/1917

Start by running the reported mypy tmp.py example with the Final attribute and conditional branches. Trace how the type checker handles the first assignment across the if and else paths; done means the command reports no reassignment error for this example and the behavior is covered by a regression test.