False positive with variable key in Mapping with Optional value

Bug Report

When using a mapping with an Optional value type, accessing a value using a variable key instead of a literal key prevents a None check from working properly.

To Reproduce

1. Put the following code in test.py:

from typing import Mapping, Optional

FOO: str = "foo"

def func(text: str) -> None:
    _ = text

data: Mapping[str, Optional[str]] = {FOO: "bar"}

if data[FOO] is not None:
    func(data[FOO])  # Line 11

if data["foo"] is not None:
    func(data["foo"])

2. Run mypy test.py

Expected Behavior

mypy should not output any errors. As the above example shows, using a string literal instead of a variable containing a string does not cause any errors, even though the code is otherwise identical. The "Optional types and the None type" section of the documents says "supported checks for guarding against a None value include if x is not None", so this code should work.

Actual Behavior

mypy outputs the following:

test.py:11: error: Argument 1 to "func" has incompatible type "Optional[str]"; expected "str"  [arg-type]

Your Environment

• Mypy version used: 0.812
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): show_error_codes = True
• Python version used: 3.7.9
• Operating system and version: macOS 10.15.7 (19H524)

Start with the reproducer in test.py and run mypy test.py to confirm the variable-key and literal-key cases differ. Trace the type-checking path for Mapping access and Optional narrowing, then add a regression test covering the variable-key case. Done means both guarded calls type-check without errors.