Eric Snow proposes that C extensions which want to be compatible with subinterpreters must use an hypothetical variant of the C API which doesn't inherit flaws of the current C API. For example, static types like "&PyLong_Type" would be excluded.

To be clear, the limited C API does expose (indirectly) "&PyLong_Type". We are talking about a new variant of the C API.

The main interpreter would continue to use its static type "&PyLong_Type", whereas each subinterpreter would get its own "int" type allocated on the heap (heap type).

Someone has to write a PoC to ensure that this idea works in practice.

In bpo-40601, I proposed that all interpreters including the main interpreter only use heap types: remove "&PyLong_Type" from the C API which is a backward incompatible C API change.