Fix strict optional with generic defaults

[hauntsaninja]

This was regressed in #21526

Fixes #21777

Co-authored-by: Codex

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

meson (https://github.com/mesonbuild/meson)
- mesonbuild/mlog.py:548:51: error: Invalid index type "slice[None, int, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:90:37: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:240:39: error: Invalid index type "slice[int, None, None]" for "str"; expected type "SupportsIndex | slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:247:45: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:250:71: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:256:45: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:259:71: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:263:71: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:265:71: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:273:53: error: Invalid index type "slice[int, None, None]" for "str"; expected type "SupportsIndex | slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:324:20: error: Invalid index type "slice[int, None, None]" for "str"; expected type "SupportsIndex | slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/programs.py:390:29: error: Invalid index type "slice[None, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/envconfig.py:480:30: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/envconfig.py:483:30: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/coredata.py:62:34: error: Invalid index type "slice[int, int, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/tooldetect.py:125:33: error: Invalid index type "slice[int, int, None]" for "str"; expected type "SupportsIndex | slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/tooldetect.py:209:37: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/environment.py:334:74: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/minstall.py:513:27: error: Invalid index type "slice[None, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/minstall.py:620:79: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mdevenv.py:130:56: error: Invalid index type "slice[int, None, None]" for "tuple[str, ...]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mdist.py:225:32: error: Invalid index type "slice[None, int, None]" for "str"; expected type "SupportsIndex | slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mdist.py:226:36: error: Invalid index type "slice[int, None, None]" for "str"; expected type "SupportsIndex | slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mesonmain.py:151:18: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mesonmain.py:183:25: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mesonmain.py:296:26: error: Invalid index type "slice[None, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mesonmain.py:305:60: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mesonmain.py:309:53: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]
- mesonbuild/mesonmain.py:322:25: error: Invalid index type "slice[int, None, None]" for "list[str]"; expected type "slice[SupportsIndex | None, SupportsIndex | None, SupportsIndex]"  [index]