Bug Report

It appears Mypy propagates types into the type arguments of a function call if the types on the left side are know, slightly obscuring the actual error.

To Reproduce

Run mypy a.py with

import typing as t

T = t.TypeVar('T')
U = t.TypeVar('U')

def coalesce(a: T | None, b: U) -> t.Union[T, U]:
  if a is None:
    return b
  return a

def foo() -> int:
  return coalesce(42, None)

Expected Behavior

Looking at this, I would expect Mypy to complain that the declared return value of foo() (being int) does not match what is actually returned (int | None).

Actual Behavior

It appears that Mypy propagates the return type into the coalesce() type arguments instead and expects them both to be int, thus complaining about the argument type instead.

a.py: note: In function "foo":
a.py:15: error: Argument 2 to "coalesce" has incompatible type "None"; expected "int"  [arg-type]
      return coalesce(42, None)
                          ^
Found 1 error in 1 file (checked 1 source file)

I find this rather unintuitive. My suggestion would be to make Mypy complain about the type of the returned value instead of the argument type.

Your Environment

• Mypy version used: 0.931

• Mypy command-line flags: /

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

  [tool.mypy]
  pretty = true
  warn_redundant_casts = true
  #warn_unused_ignores = true
  warn_no_return = true
  warn_unreachable = true
  show_error_context = true
  show_error_codes = true

• Python version used: 3.10.2

• Operating system and version: WSL 2, Ubuntu 20