self-host-check

Diff a single test between the native and self-hosted paths. A mismatch is the smoking gun for a cx.c bug.

The two commands

build/cx <file>              # native path
build/cx cx.c <file>         # self-hosted path

Run both, capture stdout and exit code for each, and diff them.

What to report

1. Native exit code and a few lines of output.
2. Self-host exit code and a few lines of output.
3. First divergence — the earliest line that differs, or the earliest assert that fails only in the self-host column.
4. A guess at which .claude/rules/cx-constraints.md pattern this looks like.

Bisecting a self-host-only failure

Once you have a minimal failing file, shrink it:

• Delete functions until the failure disappears — the last removed function (or its dependency) is the culprit.
• Inside that function, comment statements. The smallest still-failing body is the reproducer you want.
• Cross-reference the cx-constraints rules — one of them almost certainly names the pattern you just hit.

Keep the shrunken reproducer in tmp/ (gitignored) while you work. Don't add it to tests/ unless the fix lands alongside it.

Don't

• Don't check in shrunk reproducers without a fix — they become confusing failing tests nobody understands later.
• Don't conclude "self-host is broken for this language feature" without having tried a minimal repro first. Almost every time it turns out to be one of the documented patterns.