`__future__` imports aren't always eager

[brandtbucher]

Bug report

It's my understanding that __future__ imports are always supposed to be eager. PEP 810 even goes so far as to specifically exclude lazy from __future__ import ... syntax:

No, future imports can’t be lazy because they’re parser/compiler directives. It’s technically possible for the runtime behavior to be lazy but there’s no real value in it.

However, the current implementation on 3.15 and main does allow __future__ imports to become lazy when lazy imports are globally enabled:

% ./python.exe -c 'from __future__ import annotations; print(globals()["annotations"])'                      
_Feature((3, 7, 0, 'beta', 1), None, 16777216)

% PYTHON_LAZY_IMPORTS=all ./python.exe -c 'from __future__ import annotations; print(globals()["annotations"])'
<lazy_import '__future__.annotations'>

I have a patch that flags these imports as not-lazy-eligible in the bytecode compiler (similar to * imports, imports in try blocks, and non-module-level imports). Backporting this fix to 3.15 would require bumping the bytecode magic number, though (it can be checked at runtime, but that likely introduces some overhead). Just thought I'd check to make sure my understanding is correct here and that this is indeed a bug.