fix: incorrect returned type of access descriptors on unions of types

[md384]

Fixes #16603

This change maps over union types when determining the types of access descriptors. Previously, the because this conditional would fall through to the else case because instance type was not a singular TypeType (it was a Union), so we'd end up with an instance value being passed to __get__ instead of None.

[github-actions]

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

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/apply.py:265: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/apply.py:265: error: No overload variant of "__get__" of "AxisProperty" matches argument types "GroupBy[Any]", "type[GroupBy[Any]]"  [call-overload]
+ pandas/core/apply.py:265: note: Error code "call-overload" not covered by "type: ignore" comment
+ pandas/core/apply.py:265: note: Possible overload variants:
+ pandas/core/apply.py:265: note:     def __get__(self, obj: DataFrame | Series, type: Any) -> Index
+ pandas/core/apply.py:265: note:     def __get__(self, obj: None, type: Any) -> AxisProperty
+ pandas/core/apply.py:265: error: No overload variant of "__get__" of "AxisProperty" matches argument types "SeriesGroupBy", "type[SeriesGroupBy]"  [call-overload]
+ pandas/core/apply.py:265: error: No overload variant of "__get__" of "AxisProperty" matches argument types "DataFrameGroupBy", "type[DataFrameGroupBy]"  [call-overload]
+ pandas/core/apply.py:265: error: No overload variant of "__get__" of "AxisProperty" matches argument types "BaseWindow", "type[BaseWindow]"  [call-overload]
+ pandas/core/apply.py:265: error: No overload variant of "__get__" of "AxisProperty" matches argument types "Resampler", "type[Resampler]"  [call-overload]

[md384]

@JukkaL could you or another maintainer take a look at this PR?

It's (hopefully) a fairly simple fix to a fairly common sqlalchemy typing complaint.

Thanks

[hauntsaninja]

Thanks for the fix!

[aleksanb]

This PR fixed and revealed a new error with typing django models w/django-stubs.
Unsure whether to post here or on django-stubs so i'll start here on this PR:

Both Invoice and GroupInvoice here are django Models being pointed to from a third model, SalesDocument.

class SalesDocument(models.Model):
    invoice = models.OneToOneField(Invoice, on_delete=models.PROTECT, related_name='sales_document')
    group_invoice = models.OneToOneField(GroupInvoice, on_delete=models.PROTECT, related_name='sales_document')

But the following code errors out:

def foo(invoice: Invoice | GroupInvoice) -> bytes:
    invoice.sales_document  # type: ignore[call-overload]

After this patch, mypy is able to reveal the type of invoice.sales_document correctly as SalesDocument,
but errors out on the type of the overloaded sales_document method that django-stubs makes on both Invoice and GroupInvoice.

.../invoice.py:246: error: No overload variant of "__get__" of "ReverseOneToOneDescriptor" matches argument types "GroupInvoice", "type[GroupInvoice]"  [call-overload]
.../invoice.py:246: note: Possible overload variants:
.../invoice.py:246: note:     def __get__(self, instance: None, cls: Any = ...) -> ReverseOneToOneDescriptor[Invoice, SalesDocument]
.../invoice.py:246: note:     def __get__(self, instance: Invoice, cls: Any = ...) -> SalesDocument

[md384]

@aleksanb I suspect this is a django-stubs bug in the plugin code. Please file in that repo.