Infering return type as a union of all returns in type-checked functions

Feature

Note: This is different than #4409 and #6646 which aim to change what is considered "annotated" and which functions are type-checked in the first place.

Mypy currently only type-check a function if its considered "annotated", ie: all parameters are annotated, or the return type is present (required for methods w/o parameters). Whilst mypy obviously can't infer a return type if it doesn't type-check the content of a function, I feel like methods that are checked could have their return type inferred as a union of whatever mypy think the type of the returns are.

Here's a very simple example comparing mypy and pyright/pylance

(same result in CLI)

Pitch

The idea would be to reduce clutter, and reduce the risk of hiding a useful true return type behind a more vague type.

Maybe there's a performance concern for libraries? (mypy having to read more code instead of stopping at a return annotation) How much is it? How much does caching help?

No implementation files or tests are named. Start by tracing how mypy decides whether a function is type-checked and how it handles explicit return annotations, then compare that behavior with the issue's example and the referenced issues. Done should mean checked functions infer a union of their return expressions, with performance and caching implications understood.