Landing: eccc490c21fc

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Project / Subsystem

gcc / libstdc++

Date

2026-09-02

Author

Jakub Jelinek

Commit

eccc490c21fc7c9dadf8d13566963bce151c902d

Source

github

Perf win

Breaking

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All attributes

project: gcc
subsystem: libstdc++
patch_id: —
commit_hash: eccc490c21fc7c9dadf8d13566963bce151c902d
source_type: github
headline: Implement C++26 P2641R4 - Checking if a union alternative is active
tldr: Implements C++26 P2641R4 in libstdc++ for checking union active members at compile time using `__builtin_is_within_lifetime`.
author: Jakub Jelinek
outcome: committed
performance_win: false
breaking_change: false
series_id: —
series_parts: []
tags: • gcc
• libstdc++
• C++26
• compile time
• type safety
discussion_id_link: —
bugzilla_pr: —
url: https://github.com/gcc-mirror/gcc/commit/eccc490c21fc7c9dadf8d13566963bce151c902d
date: 2026-09-02T00:00:00.000Z

This patch implements the C++26 feature P2641R4, allowing compile-time checks for which member of a union is currently active. It introduces a builtin function that returns true if a given pointer points to an active union member, with specific handling for constant expressions and lifetime rules. This aligns GCC’s libstdc++ with Clang’s implementation of the paper, enhancing type safety and enabling more robust compile-time evaluations.