typing_extensions.pyi imports Protocol from typing.pyi rather than defining its own

[erictraut]

Until recently, typing_extensions.pyi declared its own Protocol symbol as follows:

Protocol: _SpecialForm = ...

It now directly imports Protocol from typing.pyi and re-exports this symbol. This means pyright can no longer differentiate between the two symbols. There's an important difference because typing.Protocol will generate a runtime exception if used in versions of Python prior to 3.7, but typing_extensions.Protocol will not.

@JelleZijlstra, is there a reason this change was made? Or put another way, is there a reason not to change it back?

Here's a link to bug that describes the regression in pyright: microsoft/pyright#2987

[JelleZijlstra]

I'm ok with changing this back; there wasn't really a compelling reason to merge the two.

[AlexWaygood]

Apologies for causing the regression @erictraut (it was my PR IIRC).

Seems like it should be added back to typing_extensions along with a comment explaining why it's duplicated there from typing rather than being re-exported, even though it's available in typing unconditionally (from typeshed's POV — at runtime, it obviously can only be imported from typing in 3.8+).