Documented @overload failes in non-stub file with --strict

[Peilonrayz]

Bug Report

The documentation for typing.overload describes how to use overloads. However when used with --strict the final, untyped, function errors. Support for typing.overload in a non-stub file has been discussed in #72 and #1136, and it looks like the documentation should work fine.

>>> import typing
>>> help(typing.overload)
Help on function overload in module typing:

overload(func)
    Decorator for overloaded functions/methods.
    
    In a stub file, place two or more stub definitions for the same
    function in a row, each decorated with @overload.  For example:
    
      @overload
      def utf8(value: None) -> None: ...
      @overload
      def utf8(value: bytes) -> bytes: ...
      @overload
      def utf8(value: str) -> bytes: ...
    
    In a non-stub file (i.e. a regular .py file), do the same but
    follow it with an implementation.  The implementation should *not*
    be decorated with @overload.  For example:
    
      @overload
      def utf8(value: None) -> None: ...
      @overload
      def utf8(value: bytes) -> bytes: ...
      @overload
      def utf8(value: str) -> bytes: ...
      def utf8(value):
          # implementation goes here

To Reproduce

Here is a playground for easy viewing.

$ mkdir example
$ cd example
$ python -m virtualenv venv
$ . venv/bin/activate.fish
$ pip install mypy
Collecting mypy
  Using cached mypy-0.790-cp38-cp38-manylinux1_x86_64.whl (22.0 MB)
Collecting typed-ast<1.5.0,>=1.4.0
  Using cached typed_ast-1.4.1-cp38-cp38-manylinux1_x86_64.whl (768 kB)
Collecting typing-extensions>=3.7.4
  Using cached typing_extensions-3.7.4.3-py3-none-any.whl (22 kB)
Collecting mypy-extensions<0.5.0,>=0.4.3
  Using cached mypy_extensions-0.4.3-py2.py3-none-any.whl (4.5 kB)
Installing collected packages: typed-ast, typing-extensions, mypy-extensions, mypy
Successfully installed mypy-0.790 mypy-extensions-0.4.3 typed-ast-1.4.1 typing-extensions-3.7.4.3
$ vim example.py
$ mypy example.py
Success: no issues found in 1 source file
$ mypy --strict example.py
example.py:9: error: Function is missing a type annotation
Found 1 error in 1 file (checked 1 source file)

example.py

from typing import overload

@overload
def utf8(value: None) -> None: ...
@overload
def utf8(value: bytes) -> bytes: ...
@overload
def utf8(value: str) -> bytes: ...
def utf8(value):
    pass

Overall

I don't think any harm will come if I silence the error with # type: ignore. However I've been really confused why it's not been working and I don't like using # type: ignore if possible. This has meant that I have opted to not use it in the past.

I fear erroring on correct usage may cause others to also not use overload.

[JelleZijlstra]

You should annotate the implementation. The example in the typing docs should perhaps also add an annotation, but that's an issue for the CPython repo, not for mypy.

[Peilonrayz]

Mypy also errors when you annotate the implementation.

from typing import overload

@overload
def utf8(value: None) -> None: ...
@overload
def utf8(value: bytes) -> bytes: ...
@overload
def utf8(value: str) -> bytes:
    pass

main.py:3: error: An overloaded function outside a stub file must have an implementation
Found 1 error in 1 file (checked 1 source file)

[JelleZijlstra]

That example has no implementation (as the error message says). It may be better to ask for help at https://gitter.im/python/typing.

[Peilonrayz]

Even if you provide an implementation you get the same error message - playground.

from typing import overload

@overload
def utf8(value: None) -> None: ...
@overload
def utf8(value: bytes) -> bytes: ...
@overload
def utf8(value: str) -> bytes:
    if value is None:
        return None
    return b''

main.py:3: error: An overloaded function outside a stub file must have an implementation
Found 1 error in 1 file (checked 1 source file)