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

[Kirito-89]

Clang's constant evaluator would unconditionally reject the initializer of any [[gnu::weak]] variable. This was intended to prevent unsafe constant folding but also incorrectly blocked valid constant evaluation in C++14 (e.g., static constexpr initialization).

This patch refines the logic to only apply the check when the evaluation mode is not EvaluationMode::ConstantExpression. This fixes the bug by allowing required constexpr evaluation to proceed while still preventing unsafe folding in other contexts.

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Adamya Verma (Kirito-89)

Changes

Clang's constant evaluator would unconditionally reject the initializer of any [[gnu::weak]] variable. This was intended to prevent unsafe constant folding but also incorrectly blocked valid constant evaluation in C++14 (e.g., static constexpr initialization).

This patch refines the logic to only apply the check when the evaluation mode is not EvaluationMode::ConstantExpression. This fixes the bug by allowing required constexpr evaluation to proceed while still preventing unsafe folding in other contexts.

---

Full diff: https://github.com/llvm/llvm-project/pull/166852.diff

1 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+1-1)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 97eeba8b9d6cc..cb3a359a6df95 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -3611,7 +3611,7 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
 
   // Never use the initializer of a weak variable, not even for constant
   // folding. We can't be sure that this is the definition that will be used.
-  if (VD->isWeak()) {
+  if (VD->isWeak()&& (Info.EvalMode != EvaluationMode::ConstantExpression)) {
     Info.FFDiag(E, diag::note_constexpr_var_init_weak) << VD;
     NoteLValueLocation(Info, Base);
     return false;