BUG: Fix check for including old cmake documentation code

[gdevenyi]

cmake CMP0106 was introduced in 3.18 not 4.0.

This fixes a bug for cmake versions between 3.18 and 4.0.

[gdevenyi]

@hjmjohnson A followup to my other work I just discovered while preparing a big build matrix of different Linux versions.

[greptile-apps]

Greptile Summary

Corrects the CMake version threshold used to decide whether to include(Documentation.cmake) from 4.0 to 3.18, which is when CMP0106 was introduced and the module was deprecated. CMake versions between 3.18 and 4.0 were incorrectly including the deprecated module, causing deprecation warnings.

• Version guard fix: CMAKE_VERSION VERSION_LESS 3.18 now accurately gates the legacy include(Documentation.cmake) path; the inline option() path is used for all CMake >= 3.18.
• Minor stale comment: The explanatory comment above the condition still refers to "CMake >= 4.0" as the cutoff, which is now inaccurate after the threshold change.

Confidence Score: 5/5

Safe to merge — this is a one-line version string fix with a well-understood and verified root cause.

The change is a precise single-line correction to a CMake version comparison. The new threshold matches when CMP0106 was introduced (3.18), the fix prevents spurious deprecation warnings on 3.18–3.x toolchains, and the only remaining concern is a now-slightly-stale inline comment.

No files require special attention beyond the already-noted stale comment in Utilities/Doxygen/CMakeLists.txt.

Important Files Changed

Filename	Overview
Utilities/Doxygen/CMakeLists.txt	Version threshold corrected from 4.0 to 3.18 so CMake 3.18–3.x users no longer trigger the deprecated Documentation.cmake include; a stale inline comment still references "CMake >= 4.0" as the inline cutoff.

_{Reviews (1): Last reviewed commit: "BUG: Fix check for including old cmake d..." | Re-trigger Greptile}

[greptile-apps]

The block comment above the condition still says "CMake >= 4.0 where the file no longer ships," but with this fix the inline option() path now activates starting at 3.18. The description is now misleading for users on 3.18–3.x, where the module still ships but is deprecated.

Suggested change

	# (preserving any auxiliary behavior it provides on era-original toolchains)
	# and inline the BUILD_DOCUMENTATION option on CMake >= 4.0 where the file
	# no longer ships.
	# (preserving any auxiliary behavior it provides on era-original toolchains)
	# and inline the BUILD_DOCUMENTATION option on CMake >= 3.18 where the module
	# is deprecated (CMP0106) or no longer ships (CMake 4.0+).

[Copilot]

Pull request overview

Fixes the CMake version gate in Utilities/Doxygen/CMakeLists.txt so ITK no longer includes CMake’s deprecated Documentation.cmake module for CMake versions where it triggers CMP0106 warnings (CMake 3.18+), while still supporting older CMake releases.

Changes:

• Update the version check to include Documentation.cmake only for CMake < 3.18 (instead of < 4.0).
• Keep BUILD_DOCUMENTATION option definition for newer CMake versions to avoid relying on the removed/deprecated module.

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[hjmjohnson]

@gdevenyi I pushed 39ecf6eae7 on top of your branch (force-with-lease) to drop the if/else entirely and keep just option(BUILD_DOCUMENTATION ...). Hope that's OK — happy to revert if you'd prefer the minimal one-line gate change.

Why drop the conditional?

The old conditional + comment block (which I had authored in c157a10 and was wrong) tried to gate the legacy include(${CMAKE_ROOT}/Modules/Documentation.cmake) so it ran on "older CMake" and the inline option() ran on "CMake ≥ 4.0 where the file no longer ships". After your gate-correction to < 3.18, the comment justification (>= 4.0 where the file no longer ships) no longer matched the actual cutoff — the real reason the inline path is taken on 3.18–3.31 is that cmake_minimum_required(VERSION 3.5...3.30) sets CMP0106 to NEW, which makes the deprecated include() a fatal error.

Once that's the rationale, the legacy branch only ever runs on CMake 3.5–3.17, and the only thing it provides that the inline path doesn't is a handful of auxiliary variables (DOCUMENTATION_HTML_HELP, DOCUMENTATION_HTML_TARZ, DOCUMENTATION_INCLUDE_TARGET_INSTALLATION). I grepped the entire release-4.14 tree — none of those are referenced anywhere — so the legacy branch was effectively a no-op. main already deleted the same conditional for the same reason.

Final form:

# CMake's Documentation.cmake (the source of BUILD_DOCUMENTATION on legacy
# toolchains) is deprecated under CMP0106 since CMake 3.18 and removed in
# CMake 4.0.  Define the option directly; no auxiliary variable from that
# module is referenced in this tree.
option(BUILD_DOCUMENTATION "Build the documentation (Doxygen)." OFF)

[gdevenyi]

This is fine. My agent also suggested this but I always want to offer minimal change solutions.