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## Summary This changeset adds proper support for assignments to attributes: ```py obj.attr = value ``` In particular, the following new features are now available: * We previously didn't raise any errors if you tried to assign to a non-existing attribute `attr`. This is now fixed. * If `type(obj).attr` is a data descriptor, we now call its `__set__` method instead of trying to assign to the load-context type of `obj.attr`, which can be different for data descriptors. * An initial attempt was made to support unions and intersections, as well as possibly-unbound situations. There are some remaining TODOs in tests, but they only affect edge cases. Having nested diagnostics would be one way that could help solve the remaining cases, I believe. ## Follow ups The following things are planned as follow-ups: - Write a test suite with snapshot diagnostics for various attribute assignment errors - Improve the diagnostics. An easy improvement would be to highlight the right hand side of the assignment as a secondary span (with the rhs type as additional information). Some other ideas are mentioned in TODO comments in this PR. - Improve the union/intersection/possible-unboundness handling - Add support for calling custom `__setattr__` methods (see new false positive in the ecosystem results) ## Ecosystem changes Some changes are related to assignments on attributes with a custom `__setattr__` method (see above). Since we didn't notice missing attributes at all in store context previously, these are new. The other changes are related to properties. We previously used their read-context type to test the assignment. That results in weird error messages, as we often see assignments to `self.property` and then we think that those are instance attributes *and* descriptors, leading to union types. Now we properly look them up on the meta type, see the decorated function, and try to overwrite it with the new value (as we don't understand decorators yet). Long story short: the errors are still weird, we need to understand decorators to make them go away. ## Test Plan New Markdown tests