Structural Pattern Matching False Positive With Nested Data Structures

[Kurt-von-Laven]

Bug Report

mypy underestimates the reachability of case blocks matching Any (or data structures containing Any) against nested data structures. This particularly impacts the use of structural pattern matching to parse JSON and other data formats that frequently include recursive data structures. It's difficult to avoid using Any for some portion of the type in these cases, so for now I believe the best workaround in these cases is to set warn_unreachable to false or suppress the mypy errors on the pertinent lines.

To Reproduce

1. Create a test.py file with the following contents:

   from typing import Any
   
   def test_structural_pattern_matching(x: Any) -> Any:
       match x:
           case {"y": [z]}:
               print("mypy believes this line is unreachable")
           case _:
               return None

2. Run mypy test.py with warn_unreachable set to true.

Expected Behavior

I expected all lines to be considered reachable since x may be {"y": [0]}, for instance.

Actual Behavior

$ mypy test.py
test.py:6: error: Statement is unreachable
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.960+dev.8faf44ad44f19c2dcf3f31f12eed5e58494fc3a3

• Mypy command-line flags: None

• Mypy configuration options from mypy.ini (and other config files):

  In pyproject.toml:

    [tool.mypy]
    warn_unreachable = true

• Python version used: 3.10.4