Overloaded functions fail when checked with --disallow-untyped-defs

[tyehle]

For this minimal example

@overload
def test(val: int) -> int: ...
@overload
def test(val: str) -> str: ...
def test(val):
    return val

with --disallow-untyped-defs mypy fails with error: Function is missing a type annotation
but without it it succeeds.

[ilevkivskyi]

This is actually expected, although somewhat unintuitive. The reason is following: currently mypy does not check the body of the function against all overloads (this is very difficult in a general case involving *args, **kwargs), overloads only provide the type/API for "external users" of a function. At the same time, mypy checks the body of a function against an (optional) type signature given in the last definition. The --disallow-untyped-defs flag prohibits unchecked function bodies, so that it errors in your case. You can fix this by writing something like this:

@overload
def test(val: int) -> int: ...
@overload
def test(val: str) -> str: ...
def test(val: Union[int, str]) -> Union[int, str]:
    return val

I just checked the documentation, and I think this is not well documented. Would you like to make a PR?

As a side comment, it is not necessary to use overloads for such simple signatures, you can just write:

T = TypeVar('T', int, str)
def test(val: T) -> T:
    return val

[tyehle]

Yeah, that was just the smallest example I could get to fail.
I'll work on a PR.

[gvanrossum]

Fixed by #3369. (If you use the exact words "Fixes #3369" in a commit message it will auto-close the issue when the PR lands.)