Banned C++ Features in Chromium

Summary

Modern C++ use in ChromiumThis document is part of the more general Chromium C++ style guide. It summarizes the supported state of new and updated language and library features in recent C++ standards and the Abseil library. This guide applies to both Chromium and its subprojects, though subprojects can choose to be more restrictive if necessary for toolchain support.The C++ language has in recent years received an updated standard every three years (C++11, C++14, etc.). For various reasons, Chromium does not immediately allow new features on the publication of such a standard. Instead, once Chromium supports the toolchain to a certain extent (e.g., build support is ready), a standard is declared “initially supported”, with new language/library features banned pending discussion but not yet allowed.You can propose changing the status of a feature by sending an email to cxx@chromium.org. Include a short blurb on what the feature is and why you think it should or should not be allowed, along with links to any relevant previous discussion. If the list arrives at some consensus, send a codereview to change this file accordingly, linking to your discussion thread.If an item remains on the TBD list two years after initial support is added, style arbiters should explicitly move it to an appropriate allowlist or blocklist, allowing it if there are no obvious reasons to ban.The current status of existing standards and Abseil features is:C++11: Default allowed; see banned features belowC++14: Default allowedC++17: Default allowed; see banned features belowC++20: Initially supported November 13, 2023; see allowed/banned/TBD features belowC++23: Initially supported January 2026; see allowed/banned/TBD features belowC++26: Not yet supportedAbseil: Default allowed; see banned/TBD features below. The following dates represent the start of the two-year TBD periods for certain parts of Abseil:absl::linked_hash_set & map: Initially added to third_party Dec 30, 2025Banned features and...