Upgrade to C++26 and enable strict compilation (-Wall -Werror)

[Luohaothu]

Summary

Upgrade C++ standard to C++26 and enable strict compiler warnings to improve code quality.

Changes

• Update C++ standard requirement from C++23 to C++26
• Enable -Wall -Werror flags in CMakeLists.txt
• Prepares codebase for warning fix PRs

Test Plan

• Build with GCC 15+
• Build with Clang 21+
• Verify all existing code compiles without warnings
• Run full test suite

Dependencies

This PR should be merged before:

• PR Impl DecayOp & Refactor ConvOp #4: Core Warning Fixes
• PR Change external dependencies management from submodule to source copy #5: Test Warning Fixes
• PR Add AMGCL solver backend #6: Clang Compatibility Fixes

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Raised the required C++ standard from C++20 to C++26 for the header-only variant.
  • Strengthened compiler warning and error checks across supported compilers, leading to stricter build-time diagnostics for downstream consumers.

[gemini-code-assist]

Summary of Changes

Hello @Luohaothu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request aims to enhance the project's code quality and future-proof its development by upgrading the C++ standard to C++26 and enforcing strict compiler warnings. This foundational change will ensure that new code adheres to modern C++ practices and that any potential issues are caught early in the development cycle, setting the stage for subsequent warning-fix pull requests.

Highlights

• C++ Standard Upgrade: The project's C++ standard requirement has been updated from C++20 to C++26 in src/CMakeLists.txt.
• Strict Compiler Warnings: Strict compiler warnings have been enabled for the opflow target. This includes -Wall, -Wextra, and -Werror for GNU and Clang compilers, and /W4, /WX for MSVC, treating warnings as errors.

Changelog

• src/CMakeLists.txt
  • Updated the required C++ standard for the opflow library from C++20 to C++26.
  • Added compiler options to enable -Wall, -Wextra, and -Werror for GNU and Clang compilers.
  • Added compiler options to enable /W4 and /WX for MSVC compilers.

Activity

• No specific activity (comments, reviews, etc.) has been recorded for this pull request yet.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The CMake header-only non-module comment was updated to reference C++26 (from C++20), and INTERFACE compiler warning flags were added to the opflow target: GNU/Clang (-Wall, -Wextra, -Werror) and MSVC (/W4, /WX).

Changes

Cohort / File(s)	Summary
Build Configuration src/CMakeLists.txt	Updated descriptive comment to reference C++26 (was C++20). Added public INTERFACE compiler warning flags for consumers of the opflow target: GNU/Clang: -Wall, -Wextra, -Werror; MSVC: /W4, /WX.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A hop, a nibble, lines refined,
The comment leaps to twenty-six in kind.
Flags raised high to catch each stray,
Warnings herd the bugs away.
I twitch my nose — the build’s polite! 🥕

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the two main changes: upgrading to C++26 and enabling strict compilation with -Wall and -Werror flags.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch build/cpp26-and-wall-werror

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[gemini-code-assist]

Code Review

This pull request upgrades the project to C++26 and enables strict compiler warnings. The changes correctly update the C++ standard version. However, there is a critical issue in the implementation of the compiler warning flags in src/CMakeLists.txt. The new code uses an undefined CMake variable, which will lead to build failures in the header-only configuration. I have provided a detailed comment with a code suggestion to resolve this issue.

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: ff28310f56

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In `@src/CMakeLists.txt`:
- Around line 125-130: The CMakeLists uses OPFLOW_TARGET_SCOPE in
target_compile_options(opflow ${OPFLOW_TARGET_SCOPE} ...) but
OPFLOW_TARGET_SCOPE is never defined; define it (e.g., set(OPFLOW_TARGET_SCOPE
INTERFACE) or PUBLIC/PRIVATE as appropriate) before its use or remove the
variable and substitute the correct scope keyword directly where
target_compile_options is called (refer to OPFLOW_TARGET_SCOPE and the
target_compile_options(opflow ...) invocation to locate the change).
- Around line 45-50: The project adds an INTERFACE target opflow and requires
C++26 via target_compile_features(opflow INTERFACE cxx_std_26), so update CI to
guarantee compilers that support C++26 (GCC >=14, Clang >=18, MSVC >=17.13) or
add an explicit pre-check in CMake to detect compiler versions and emit a clear
error; specifically adjust your CI job definitions (toolchain images/versions)
to use the stated minimums and/or implement a CMake guard around
add_library/opflow and target_compile_features that tests CMAKE_CXX_COMPILER_ID
and CMAKE_CXX_COMPILER_VERSION and fails with a helpful message if the compiler
is too old.

🧹 Nitpick comments (1)

src/CMakeLists.txt (1)

123-137: Avoid exporting -Werror//WX to consumers by default.

Using -Werror (or /WX) on an INTERFACE/PUBLIC target can break downstream builds due to their own warnings. Consider limiting Werror to build-interface or making it opt‑in.

Proposed adjustment (limit Werror to build interface)

 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     target_compile_options(opflow ${OPFLOW_TARGET_SCOPE}
         -Wall
         -Wextra
-        -Werror
+        "$<$<BUILD_INTERFACE:-Werror>>"
     )
 endif()
 # MSVC-specific warnings
 if(MSVC)
     target_compile_options(opflow ${OPFLOW_TARGET_SCOPE}
         /W4
-        /WX
+        "$<$<BUILD_INTERFACE:/WX>>"
     )
 endif()