ADR-0004: C++23 toolchain & build

Status: Accepted (2026-06-09).

Context

Observed dev box (2026-06-09): GCC 16.1 (system g++), g++-15 present, CMake 3.31, Ninja 1.13, kernel 6.17; clang absent. std::expected, std::print, and std::format all confirmed working under -std=c++23. liburing not installed.

Decision

• Language: C++23. Compiler floor: GCC 14+ (or Clang 18+); we develop on GCC 16.
• Build: CMake ≥ 3.28, Ninja generator. Dev builds use -Wall -Wextra -Wpedantic -Werror and ASan/UBSan in Debug.
• Headers only for now — C++ modules OFF. Module/toolchain maturity isn’t worth the build risk yet.
• Dependencies: OpenSSL — required (libssl-dev; TLS for the runtime-optional HTTPS listener, ADR-0005, and the long-input SHA-256 fallback where selected; short-key scalar and hardware SHA implementations are vendored, ADR-0014). liburing — required for the io_uring backend (liburing-dev; detected via pkg-config, gated by GOBLIN_HAVE_URING; the pure-logic layers build without it, but the server can’t serve). HTTP/2,3 later: nghttp2 / ngtcp2 + nghttp3.
• C++23 features we use now: std::expected (hot-path error returns, no exceptions), std::span, std::print/std::format, if consteval + constexpr (compile-time command table). Defer std::generator, std::flat_map, std::mdspan until measured.

Consequences

• ➕ Modern, exception-light error handling; fast iteration with Ninja.
• ➖ The GCC-14 floor excludes some older LTS distros — acceptable; document the install.