Summary

• Add C++20 module interface files (.cppm) for common_system as part of Epic [EPIC] C++20 Module Migration - Cross-System Preparation #256 Phase 2
• Create module structure under src/modules/ with partitioned architecture
• Update CMakeLists.txt with optional module build target
• Document changes in CHANGELOG.md

Changes

Module Structure

src/modules/
├── common.cppm              # Primary module (import kcenon.common;)
├── utils.cppm               # CircularBuffer, ObjectPool, source_location
├── error.cppm               # Error codes and categories
├── result.cppm              # Result aggregator
├── result/
│   ├── core.cppm            # Result<T>, Optional<T>, error_info
│   └── utilities.cppm       # Factory functions, exception conversion
├── concepts.cppm            # C++20 type constraints
├── interfaces.cppm          # Interfaces aggregator
├── interfaces/
│   └── core.cppm            # IExecutor, ILogger, IThreadPool
├── config.cppm              # Configuration utilities
├── di.cppm                  # Dependency injection
├── patterns.cppm            # EventBus implementation
└── logging.cppm             # Logging utilities

CMake Changes

• Updated minimum CMake version to 3.28 for module support
• Added COMMON_BUILD_MODULES option (default: OFF)
• Added common_system_modules target with target-specific module scanning
• Module scanning only applied to module target (avoids breaking CMake internal try_compile)

Test plan

• Confirm backward compatibility with header includes (CI passed on all platforms)
• Test on multiple compilers: ubuntu-22.04/clang, ubuntu-22.04/gcc, macos-latest/clang, windows-2022/msvc
• All sanitizers passed (address, thread, undefined)
• Static analysis passed (Clang-Tidy, Cppcheck)
• Warning checks passed (GCC, Clang)
• Build with modules enabled: cmake -DCOMMON_BUILD_MODULES=ON .. (optional feature, requires Phase 3)
• Verify import kcenon.common; works correctly (Phase 3: Module Consumption Tests)
• Verify individual partition imports work (Phase 3: Module Consumption Tests)

Note: Module build testing (COMMON_BUILD_MODULES=ON) is planned for Phase 3 (#266) which adds test examples. This PR focuses on creating the module interface files with backward-compatible default behavior.

Related

• Closes [MODULE] Create C++20 module files for common_system - Phase 2 #265
• Part of Epic [EPIC] C++20 Module Migration - Cross-System Preparation #256 (C++20 Module Migration)
• Next: Phase 3 ([CLEANUP] Remove deprecated Result headers - v1.0.0 preparation #266) - Module Consumption Tests