False-positive when using Union[Tuple[...]]

Bug Report / To Reproduce

I have a incorrect Unsupported operand types for + ("int" and "str") when using Union[Tuple[...]].

To Reproduce

Have a look at this example:

from typing import Union, Tuple, List

variable: List[Union[Tuple[int, int], Tuple[str, str]]] = []

element1 = variable[0]

print(element1[0] + element1[1])

Run mypy.

Expected Behavior

No warning.

Actual Behavior

I have:

Unsupported operand types for + ("int" and "str")

But it's wrong due to the fact that element1 is a Union (right) but it's the same variable on both side. So both type on the left and the right of the "+" should be the same.

Your Environment

• Mypy version used: mypy 0.790
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
python_version = 3.8
ignore_missing_imports = True
follow_imports = silent
show_column_numbers = True
disallow_untyped_calls = True
disallow_untyped_defs = True
disallow_incomplete_defs = True
check_untyped_defs = True
no_implicit_optional = True
warn_redundant_casts = True
warn_unused_ignores = True
warn_unreachable = True
strict_equality = True

• Python version used: Python 3.7.3
• Operating system and version: Windows 10 20H2 (19042.630)

Start by running mypy on the reported Python reproducer with the stated Python 3.8 configuration and confirm the false positive. Trace how the type checker handles repeated indexing of a Union of tuples, then verify that the example produces no warning while preserving correct errors for genuinely incompatible operands.