llvm/llvm-project

`[[gnu::weak]]` disables constant evaluation of constexpr variables in C++14

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#141,797 opened on May 28, 2025

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c++11c++14clang:frontenddiverges-from:gccgood first issue

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Description

GCC and Clang disagree on their handling of [[gnu::weak]] before C++17 https://gcc.godbolt.org/z/s7q67vqjz

struct S { static constexpr int var = 0; };
[[gnu::weak]] constexpr int S::var;
static constexpr int usage = S::var;

While it would be unsafe to constant-fold this code, it should be fine to constant-evaluate it, for consistency with GCC

In static bool evaluateVarDeclInit (ExprConstant.cpp), and in static bool CheckWeak (Interp.cpp), we should allow constant evaluation -without diagnostics, but not constant folding) - by looking at Info.EvalMode

Contributor guide

Research direction
Examine the functions `evaluateVarDeclInit` in ExprConstant.cpp and `CheckWeak` in Interp.cpp. Focus on `Info.EvalMode` to allow constant evaluation without diagnostics while preventing constant folding.
Tech stack
cpp
Domain
compilers
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
C++compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50