OPA Rego Compatibility Check

Determine what changes (if any) rego-cpp needs to maintain compatibility with the latest OPA Rego release.

When to Use

• Checking if a new OPA version has been released since rego-cpp's last update
• Planning work to upgrade rego-cpp to a newer OPA version
• Auditing the current compatibility gap between rego-cpp and OPA
• Reviewing what changed in OPA that affects rego-cpp

Procedure

Follow these steps in order. Do not skip steps.

Step 1: Determine Current rego-cpp OPA Version

Read the REGOCPPOPAVERSION variable from the root CMakeLists.txt:

grep "REGOCPP_OPA_VERSION" CMakeLists.txt

This is the OPA version rego-cpp currently targets. Record it as CURRENT_VERSION.

Also read the VERSION file in the repo root to get the rego-cpp library version.

Step 2: Fetch Latest OPA Release Information

Fetch the OPA releases page to find the latest version and release notes:

1. Fetch https://github.com/open-policy-agent/opa/releases with query "latest release version"
2. Identify the latest stable release version. Record it as LATEST_VERSION.
3. If the user specified a target version, use that instead of the latest.

If LATESTVERSION equals CURRENTVERSION, report that rego-cpp is up to date and stop.

Step 3: Fetch Release Notes for Each Intermediate Version

For each OPA version between CURRENTVERSION (exclusive) and LATESTVERSION (inclusive):

1. Fetch https://github.com/open-policy-agent/opa/releases/tag/v{VERSION} with query "release notes changes new features built-ins deprecations breaking changes Rego language"
2. Also fetch https://www.openpolicyagent.org/docs/latest/policy-reference/ with query "built-in functions list" to get the current built-in function catalog.

Collect all changes across versions.

Step 4: Categorize Changes by Impact Area

Sort every change into the categories below. Refer to [change-surface.md](./references/change-surface.md) for details on how each category maps to rego-cpp code.

Step 5: Cross-reference with rego-cpp Built-in Coverage

For any new or modified built-ins, check whether rego-cpp already implements them:

1. Search src/builtins/ for the function name
2. Check src/builtins.cc lookup dispatch to see if the namespace is routed
3. Check src/builtins/builtins.hh for the namespace factory declaration

Mark each new built-in as one of:

• Already implemented — no action needed
• Namespace exists, function missing — add to existing file
• New namespace — new file needed in src/builtins/, plus registration

See [builtin-pattern.md](./references/builtin-pattern.md) for the implementation pattern.

Step 6: Produce the Compatibility Report

Generate a structured report with the following sections:

# OPA Rego Compatibility Report

## Version Summary
- **rego-cpp version**: {from VERSION file}
- **Current OPA target**: {CURRENT_VERSION}
- **Latest OPA release**: {LATEST_VERSION}
- **Versions to bridge**: {list of intermediate versions}

## Required Changes

### New Built-in Functions
For each new built-in:
- Function signature (name, args, return type)
- Which `src/builtins/` file to modify or create
- Complexity estimate (trivial/moderate/complex)

### Built-in Semantic Changes
For each changed built-in:
- What changed and how
- Which file to modify

### Deprecations
For each deprecated item:
- Add to `is_deprecated()` list in `src/builtins.cc`

### Language Changes
For each syntax/grammar change:
- What changed in the grammar
- Parser modifications needed
- New tokens needed in `include/rego/rego.hh`
- Documentation updates needed in `README.md` grammar section

### Evaluation Changes
For each evaluation behavior change:
- Description of change
- Affected compiler passes or VM behavior

### Bundle Format Changes
For each format change:
- What changed
- Files to update

## Version Bump Checklist
- [ ] Update `REGOCPP_OPA_VERSION` in `CMakeLists.txt`
- [ ] Update OPA version reference in `README.md`
- [ ] Sync `README.md` grammar section with OPA grammar
- [ ] Update OPA version reference and grammar in wrapper docs:
  - [ ] `wrappers/rust/regorust/README.md` ("We support v… of Rego" line; grammar is deferred to the main README)
  - [ ] `wrappers/python/README.md` ("We support v… of Rego" line; grammar is deferred to the main README)
  - [ ] `wrappers/dotnet/docs/introduction.md` ("largely compatible with v…" line)
- [ ] Implement new built-ins (list each)
- [ ] Update `is_deprecated()` for newly deprecated built-ins
- [ ] Apply parser changes (if any)
- [ ] Apply evaluation changes (if any)
- [ ] Run OPA conformance tests: `cmake --preset debug-clang-opa && ninja -C build && ctest --test-dir build -R opa`
- [ ] Update `CHANGELOG` with new version entry
- [ ] Update `VERSION` file

## No Impact (documented for completeness)
- List of OPA changes that don't affect rego-cpp

Step 7: Analyze OPA IR for Complex Features

For any language syntax changes or new internal built-ins identified in Step 4, use OPA's plan IR to understand the implementation before coding:

1. Ensure latest OPA binary is available:

``bash curl -L -o /tmp/opa https://github.com/open-policy-agent/opa/releases/download/v{LATESTVERSION}/opalinuxamd64static chmod +x /tmp/opa /tmp/opa version # verify it matches LATEST_VERSION ``

1. Create minimal test policies in .copilot/opa-ir-test/, one per feature:

``bash mkdir -p .copilot/opa-ir-test cat > .copilot/opa-ir-test/policy.rego << 'EOF' package test p := <feature expression using both constants and variables> EOF ``

1. Build and inspect the IR:

``bash /tmp/opa build --bundle .copilot/opa-ir-test --target plan -e test/p -o .copilot/opa-ir-test/bundle.tar.gz mkdir -p .copilot/opa-ir-test/output && cd .copilot/opa-ir-test/output tar xzf ../bundle.tar.gz && python3 -m json.tool plan.json ``

1. Look for:

- New entries in static.builtin_funcs — these are internal built-ins rego-cpp must implement - The exact calling convention (arg types, arity, return type) - How undefined values are handled (e.g., BlockStmt + Set wrapping patterns) - Whether OPA's optimizer folds constant cases differently from variable cases (test both)

1. Record findings in the compatibility report under the relevant section.

See [change-surface.md § Analyzing OPA's IR](./references/change-surface.md) for detailed examples.

Step 8: Validate Findings

If the workspace has a build directory and the OPA test suite available:

1. Remove the stale OPA clone before reconfiguring: rm -rf build/opa
2. Reconfigure: cmake .. --preset debug-clang-opa (this re-clones at the new version tag)
3. Run targeted tests for specific areas during development:

``bash ./tests/rego_test -wf opa/v1/test/cases/testdata/v1/<subdir> ` Subdirectory names match OPA built-in names without separators (e.g., regexfind, numbersrangestep, stringinterpolation`).

1. Run the full conformance suite for final validation:

``bash ctest -R regotestopa --output-on-failure ``

Important Notes

• OPA release notes are at https://github.com/open-policy-agent/opa/releases
• OPA built-in reference is at https://www.openpolicyagent.org/docs/latest/policy-reference/
• The OPA conformance test suite is cloned from https://github.com/open-policy-agent/opa/ at the tag matching REGOCPPOPAVERSION
• rego-cpp only implements the Rego language and built-in functions. Changes to OPA's Go runtime, REST API, plugin system, or CLI tool do not apply.
• Some built-ins may be platform-dependent (e.g., time zone functions require cpplibchrono >= 201907L). Flag these in the report.
• http.send is a stub in rego-cpp — new HTTP-related functionality is typically marked as a placeholder.