Overly restrictive narrowing of TypeVar via isinstance

[huguesb]

Bug Report

When a value whose type is a TypeVar is tested via isinstance, mypy so aggressively narrows types that it refuses to let them be broadened back to the original TypeVar, even when the value is untouched.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.10&gist=ac798d8bf4d5a39d1446d86bd5c30cf3

from typing import TypeVar

T = TypeVar('T')

def foo(t: T) -> T:
  if isinstance(t, str):
      return t + "."
  elif isinstance(t, dict):
      return t
  elif isinstance(t, list):
      return t
  else:
    return t

Expected Behavior

no type errors

Actual Behavior

main.py:8: error: Incompatible return value type (got "str", expected "T")
main.py:10: error: Incompatible return value type (got "Dict[Any, Any]", expected "T")
main.py:12: error: Incompatible return value type (got "List[Any]", expected "T")
Found 3 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.971 (reproduces in all versions I've tried in the playground)
• Mypy command-line flags: N/A
• Python version used: 3.7 (reproduces in all versions I've tried in the playground)

[tmke8]

Same as #12800?

[kmillikin]

The second and third errors do look like bugs. The type of t after isinstance(t, dict) should be revealed as <subclass of "T" and "Dict[Any, Any]"> and it should be OK to return this as a value of type T.

The first one is more subtle. After isinstance(t, str) the type of t should be revealed as <subclass of "T" and "str">. But the string concatenation operator returns a str and T might actually be a subtype of str, so it's not necessarily OK to return that as a value of type T.

[hauntsaninja]

Closing as duplicate of #10817