[clang] #141797 issue fixed (PR #166852)

[Adamya Verma via cfe-commits]

https://github.com/Kirito-89 created https://github.com/llvm/llvm-project/pull/166852

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.

>From 253268dabad5b5d0ccba15ad1d532fda0916a1a2 Mon Sep 17 00:00:00 2001
From: Adamya Verma <cs23btech11001 at iith.ac.in>
Date: Fri, 7 Nov 2025 03:09:37 +0530
Subject: [PATCH] #141797 issue fixed

---
 clang/lib/AST/ExprConstant.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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;