qt-migration-copilot-demo/.github/copilot-instructions.md

GitHub Copilot Repository Instructions: C++23 Modernization

Context

• Language: C++
• Target standard: C++23
• Goal: modernize legacy C++11/14/17 code safely and incrementally

Primary Rule

Preserve existing behavior unless functional changes are explicitly requested.

Balanced Modernization Policy

• Prefer modern C++23 constructs when they clearly improve safety, readability, or maintainability.
• Keep refactors focused and reviewable; avoid broad rewrites unless requested.
• Prefer standard library solutions over custom or legacy patterns.
• Respect toolchain limitations, ABI-sensitive interfaces, and repository-local style.

Preferred Modernizations

• Use auto where the type is obvious from the initializer; avoid it when it hides important type information.
• Prefer structured bindings over std::tie or manual pair/tuple access when clearer.
• Replace raw owning pointers and manual delete with RAII, usually std::unique_ptr.
• Use std::make_unique and std::make_shared; prefer std::shared_ptr only for real shared ownership.
• Use std::string_view for read-only string parameters only when lifetime safety is clear.
• Prefer std::span for non-owning contiguous ranges.
• Use std::mdspan only when multidimensional access is central and the change improves clarity or safety.
• Prefer .contains() over find(...) != end() where available and clearer.
• Prefer standard algorithms and ranges over manual loops when readability improves.
• Use [[nodiscard]] where ignoring the result is likely a bug.
• Prefer early returns over deeply nested control flow.
• Use std::optional monadic operations (and_then, transform, or_else) when they make the code simpler.
• Prefer constexpr, if constexpr, concepts, and requires over legacy std::enable_if / SFINAE where practical.
• Add noexcept, override, final, = default, and = delete where correct and useful.
• Prefer enum class over unscoped enums where practical.
• Prefer std::print / std::println for console output when supported by the project toolchain.
• Prefer typed std::chrono durations and std::jthread where appropriate.

Avoid

• Do not modernize mechanically if readability becomes worse.
• Do not introduce unsupported C++23 features.
• Do not introduce dangling references, dangling std::string_view, or unclear ownership.
• Do not silently change ABI, object lifetime, exception behavior, or performance characteristics.
• Do not rewrite unrelated code.

Expected Copilot Behavior

When making migration suggestions:

• produce small, focused updates
• explain non-trivial changes briefly
• call out risky refactors explicitly
• note compatibility or toolchain caveats
• leave optional follow-up improvements separate

Preferred Output

Provide:

• updated code
• short rationale
• compatibility/toolchain caveats
• optional next modernization steps