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 se nombran archivos de implementación ni pruebas. Empieza por rastrear cómo mypy decide si una función se comprueba en cuanto a tipos y cómo gestiona las anotaciones de retorno explícitas; después, compara ese comportamiento con el ejemplo del issue y los issues referenciados. Se considerará terminado cuando las funciones comprobadas infieran una unión de sus expresiones de retorno y se comprendan las implicaciones para el rendimiento y el almacenamiento en caché.