build(engine): enforce C++17 in CMake and fix Data target standard

[tianjianjiang]

Summary

• Add CMAKE_CXX_STANDARD_REQUIRED ON and CMAKE_CXX_EXTENSIONS OFF to all three CMakeLists.txt files (Engine, gramambular2, Mandarin) to enforce strict C++17 compliance
• Change Data target from gnu++0x (C++11 with GNU extensions) to c++17 in both Debug and Release configurations in project.pbxproj

Independent PR (base: master). Previously part of the contextual user model stack; detached as it stands alone.

Test plan

• C++ engine tests: 100/100 passed
• grep gnu++0x project.pbxproj returns no matches
• grep STANDARD_REQUIRED returns 3 hits (one per CMakeLists.txt)
• Xcode build: pre-existing SPM dependency failure (same on master, unrelated)

Generated with Claude Code

[claude]

The changes correctly enforce C++17 standard compliance across all build configurations. No critical issues found - the PR properly:

• Adds CMAKE_CXX_STANDARD_REQUIRED ON to fail builds if C++17 is unavailable
• Adds CMAKE_CXX_EXTENSIONS OFF to disable compiler-specific extensions (e.g., GNU extensions)
• Upgrades Data target from gnu++0x (C++11 with GNU extensions) to c++17

This aligns with CLAUDE.md requirements and prevents accidental use of C++20/C++23 features.

[gemini-code-assist]

Code Review

This pull request aims to enforce strict C++17 compliance across the project. The changes correctly update the CMake build system to require C++17 and disable compiler extensions. The Xcode project's Data target is also updated from C++11 with GNU extensions to C++17.

My review identifies some redundant build settings in the subdirectory CMakeLists.txt files. These settings are already defined in the parent CMakeLists.txt and are inherited, so they can be removed from the subdirectory files to improve maintainability. I've added comments to the newly added redundant lines.

Overall, the changes are good and align with the goal of standardizing the C++ version.

[Copilot]

Pull request overview

This pull request enforces strict C++17 compliance across both the CMake-based C++ engine build and the Xcode project build system. Building on the guardrails established in #783, it eliminates outdated C++11 settings and disables GNU extensions to ensure consistent, standards-compliant C++ compilation.

Changes:

• Add CMAKE_CXX_STANDARD_REQUIRED ON and CMAKE_CXX_EXTENSIONS OFF to all three CMakeLists.txt files for strict C++17 enforcement
• Update Data target from obsolete gnu++0x (C++11 with GNU extensions) to c++17 in Xcode project

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Source/Engine/CMakeLists.txt	Add strict C++17 enforcement flags (STANDARD_REQUIRED ON, EXTENSIONS OFF) to main engine CMake configuration
Source/Engine/gramambular2/CMakeLists.txt	Add strict C++17 enforcement flags to gramambular2 library CMake configuration
Source/Engine/Mandarin/CMakeLists.txt	Add strict C++17 enforcement flags to Mandarin library CMake configuration
McBopomofo.xcodeproj/project.pbxproj	Update Data target from gnu++0x to c++17 in both Debug and Release configurations

[claude]

No critical issues found. Changes correctly enforce strict C++17 compliance across CMake and Xcode build systems.

[claude]

The changes correctly enforce strict C++17 compliance across all CMake files and update the Data target from outdated C++11 with GNU extensions to C++17. The implementation is sound.

One note: verify that the Data target builds successfully with the stricter standard, as it previously used GNU extensions.

[claude]

No critical issues found. The changes properly enforce C++17 standard compliance across all CMake build files and update the Data target from gnu++0x (C++11 with GNU extensions) to c++17, matching the repository's C++17 requirement documented in CLAUDE.md.

[claude]

Reviewed changes for C++17 standard enforcement. Found one critical CMake configuration issue that could prevent proper standard enforcement in subdirectories. See inline comment for details.

[claude]

No critical issues found. The changes correctly enforce C++17 compliance across all build systems as specified in CLAUDE.md.

[claude]

Reviewed the diff and verified the project.pbxproj context. The two changed XCBuildConfiguration objects (6A38BC23 / 6A38BC24) belong to PBXLegacyTarget "Data" only — not project-wide — so the scope matches the PR description.

No critical issues found:

• CMakeLists.txt (Engine, gramambular2, Mandarin): CMAKE_CXX_STANDARD_REQUIRED ON and CMAKE_CXX_EXTENSIONS OFF are correct additions. The former turns a silent compiler downgrade into a hard error; the latter enforces portable ISO C++17 (i.e., -std=c++17 instead of -std=gnu++17). Both are safe on macOS/Clang, and all 100 engine tests continue to pass.

• project.pbxproj: gnu++0x (C++11 + GNU extensions) → c++17 for the Data target is the right fix to align it with the rest of the project. Note that Data is a PBXLegacyTarget that runs a Makefile/shell script rather than a Clang compilation phase, so CLANG_CXX_LANGUAGE_STANDARD may not be forwarded to the build tool as a compiler flag. The setting is harmless and documents intent, but the Makefile in Source/Data/ is the authoritative place where any C++ compiler invocations would need to respect this standard.