Further improve return types in the numbers module

[AlexWaygood]

Followup to #11353 (fixes 2/3 new errors in #11353 (comment))

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pytest (https://github.com/pytest-dev/pytest)
- src/_pytest/python_api.py:453: error: Argument 1 to "abs" has incompatible type "SupportsComplex"; expected "SupportsAbs[Never]"  [arg-type]

ibis (https://github.com/ibis-project/ibis)
- ibis/util.py:168: error: Unsupported left operand type for < (Never)  [operator]
+ ibis/util.py:168: error: Unsupported left operand type for < ("_RealLike")  [operator]
- ibis/util.py:168: error: Argument 1 to "abs" has incompatible type "SupportsComplex"; expected "SupportsAbs[Never]"  [arg-type]

[JelleZijlstra]

Why don't you make these two return RealLike?

[AlexWaygood]

I tried doing this:

--- a/stdlib/numbers.pyi
+++ b/stdlib/numbers.pyi
@@ -41,8 +41,8 @@ else:
 # as subtypes of _RealLike have a `__complex__` method
 # (e.g. `builtins.int.__complex__` does not exist)
 class _RealLike(Protocol):
-    def __neg__(self) -> _ComplexLike: ...  # TODO: ideally would be more precise
-    def __pos__(self) -> _ComplexLike: ...  # TODO: ideally would be more precise
+    def __neg__(self) -> _RealLike: ...
+    def __pos__(self) -> _RealLike: ...
     def __abs__(self) -> _RealLike: ...
     def __float__(self) -> float: ...
     def __trunc__(self) -> _IntegralLike: ...
@@ -143,6 +143,12 @@ class Real(Complex):
     @property
     def imag(self) -> Literal[0]: ...
     def conjugate(self) -> _RealLike: ...  # type: ignore[override]
+    # Not actually overridden at runtime,
+    # but we override these in the stub to give them more precise return types:
+    @abstractmethod
+    def __pos__(self) -> _RealLike: ...
+    @abstractmethod
+    def __neg__(self) -> _RealLike: ...

But on 3.12, mypy complains:

stdlib\numbers.pyi:149: error: Return type "_RealLike" of "__pos__" incompatible with return type "_SupportsComplex" in supertype "Complex"  [override]
stdlib\numbers.pyi:151: error: Return type "_RealLike" of "__neg__" incompatible with return type "_SupportsComplex" in supertype "Complex"  [override]

And on 3.8, mypy complains with:

stdlib\numbers.pyi:149: error: Return type "_RealLike" of "__pos__" incompatible with return type "Union[_SupportsComplex, complex]" in supertype "Complex"  [override]
stdlib\numbers.pyi:151: error: Return type "_RealLike" of "__neg__" incompatible with return type "Union[_SupportsComplex, complex]" in supertype "Complex"  [override]

I think this is because _RealLike does not have a __complex__ method, so isn't seen as a subtype of _SupportsComplex. (It can't have a __complex__ method, because int doesn't have a __complex__ method, and we need int to be understood as a subtype of RealLike.)

This is yet another way in which these ABCs are subtly broken, unfortunately. Another approach might be to return SupportsAbs[_RealLike] | complex (instead of _SupportsComplex | complex) from most of the numbers.Complex methods.

[AlexWaygood]

Closing in favour of #11375