"An implementation for an overloaded function is not allowed in a stub file" for overloaded type hint

Bug Report

I'm documenting a compiled extension module written in Rust. I write docstrings in my .pyi file so that IDEs and API documentation can automatically pick up those docstrings.

Docstrings are allowed in functions but this errors when a function is overloaded.

To Reproduce

# tmp.pyi

from typing import overload

def fn1() -> None:
    """Call this function."""

@overload
def fn2(a: str) -> str: ...
@overload
def fn2(a: int) -> int: ...
def fn2(a: int | str) -> int | str:
    """Call this function."""

uv run mypy tmp.pyi

gives:

tmp.pyi:10: error: An implementation for an overloaded function is not allowed in a stub file  [misc]

Note that this does not error on line 3 for the non-overloaded function.

Expected Behavior

Expected mypy to not error when a docstring is included on a function that has overloads.

Actual Behavior

tmp.pyi:10: error: An implementation for an overloaded function is not allowed in a stub file  [misc]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used:

  > uv run mypy --version
  mypy 1.15.0 (compiled: yes)
  

• Mypy command-line flags: None, just uv run mypy tmp.pyi

• Mypy configuration options from mypy.ini (and other config files): None

• Python version used:

  > uv run python --version
  Python 3.9.21
  

Start by running uv run mypy tmp.pyi with the reproducer and trace the diagnostic reported on line 10 for the overloaded stub function. Add regression coverage for allowing its docstring, and confirm the command completes without that error.