Fail fast for system SWIG 4.4+ with Python limited API

[SID-6921]

Summary

• add a configure-time guard for the known-broken combination:
  • ITK_USE_SYSTEM_SWIG=ON
  • ITK_WRAP_PYTHON=ON
  • ITK_USE_PYTHON_LIMITED_API=ON
  • SWIG_VERSION >= 4.4.0
• emit a clear actionable error instead of allowing a build that fails later at Python import time

Why

Issue #6479 documents that this combination can compile/link but then fails at import with missing delete_SwigPyIterator from the generated limited-API wrapper module. This change prevents users from reaching that late failure mode.

User guidance in error

The message recommends one of:

• use system SWIG 4.3.x
• set ITK_USE_PYTHON_LIMITED_API=OFF
• or set ITK_USE_SYSTEM_SWIG=OFF

Testing

• CMake configure logic change only (no runtime behavior change outside guarded configuration path)
• patch validated by source inspection and targeted diff.

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[Copilot]

Pull request overview

Adds a CMake configure-time guard to fail fast when configuring ITK Python wrapping with the Python Limited API while using a system SWIG version known to generate wrappers that later fail at import time (SWIG ≥ 4.4.0). This prevents late runtime import errors by providing an early, actionable configuration error.

Changes:

• Add a FATAL_ERROR configure-time check for ITK_USE_SYSTEM_SWIG=ON + ITK_WRAP_PYTHON=ON + ITK_USE_PYTHON_LIMITED_API=ON when SWIG_VERSION >= 4.4.0.
• Provide user guidance in the error text with suggested configuration alternatives (use SWIG 4.3.x, disable Limited API, or disable system SWIG).

---

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[greptile-apps]

Greptile Summary

This PR adds a configure-time guard for a known-broken SWIG and Python limited API setup. The main changes are:

• Rejects system SWIG when Python wrapping and the limited API are enabled with SWIG_VERSION >= 4.4.0.
• Emits a clear CMake fatal error before the build reaches import-time wrapper failures.
• Suggests using system SWIG 4.3.x, disabling the Python limited API, or using ITK-managed SWIG.

Confidence Score: 4/5

Safe to merge after the minor comment mismatch is addressed.

The functional CMake guard is isolated to configure-time validation and matches the PR intent. The only finding is non-blocking documentation drift in the new in-source comment.

Wrapping/Generators/SwigInterface/CMakeLists.txt

T-Rex Logs

What T-Rex did

• Ran the base CMake configure to verify the SWIG limited API guard; it found fake SWIG 4.4.0 and completed with exit code 0.
• Ran the head CMake configure for the SWIG limited API guard; it found fake SWIG 4.4.0 and failed at Wrapping/Generators/SwigInterface/CMakeLists.txt:196, with the documented guidance, and the included negative control completed with exit code 0 when limited API was OFF.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
Wrapping/Generators/SwigInterface/CMakeLists.txt	Adds a configure-time fatal error for system SWIG 4.4+ with Python limited API; only issue found is a comment that says 4.4.x while the guard covers 4.4+.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
A[Configure SwigInterface] --> B{ITK_USE_SYSTEM_SWIG}
B -->|ON| C[find_package SWIG]
C --> D{SWIG_VERSION < minimum}
D -->|yes| E[FATAL_ERROR: newer SWIG required]
D -->|no| F{Python wrapping + limited API + SWIG >= 4.4.0}
F -->|yes| G[FATAL_ERROR: unsupported combination]
F -->|no| H[Continue configure]
B -->|OFF and prebuilt| I[Use prebuilt SWIG path]

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
A[Configure SwigInterface] --> B{ITK_USE_SYSTEM_SWIG}
B -->|ON| C[find_package SWIG]
C --> D{SWIG_VERSION < minimum}
D -->|yes| E[FATAL_ERROR: newer SWIG required]
D -->|no| F{Python wrapping + limited API + SWIG >= 4.4.0}
F -->|yes| G[FATAL_ERROR: unsupported combination]
F -->|no| H[Continue configure]
B -->|OFF and prebuilt| I[Use prebuilt SWIG path]

Loading

_{Reviews (1): Last reviewed commit: "Fail fast for system SWIG 4.4+ with Pyth..." | Re-trigger Greptile}

[greptile-apps]

Clarify blocked versions
The guard blocks every SWIG_VERSION VERSION_GREATER_EQUAL 4.4.0, but the comment narrows the rationale to 4.4.x. That mismatch can mislead future maintainers into thinking 4.5.0 and later should not be rejected even though the condition intentionally fails fast for them.

Context Used: AGENTS.md (source)

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[SID-6921]

Applied the wording nit from review: updated the in-file note to say SWIG 4.4+ (matching the VERSION_GREATER_EQUAL 4.4.0 guard) in commit 4b11d44.

[thewtex]

@SID-6921 thanks for the contribution! 🙏

[dzenanz]

Preferably squash upon merge.

[dzenanz]

Or even better, squash into one commit now.

[SID-6921]

Done. I squashed the branch into a single commit as requested and force-pushed: 3eccfe7.