[clang] [clang][bytecode] Avoid crash in constexpr wcslen on invalid argument… (PR #177891)

[Ayush Kumar Gaur via cfe-commits]

https://github.com/Ayush3941 updated https://github.com/llvm/llvm-project/pull/177891

>From 7679af9795c8a9769e4e2a298ca4ed9db5183ee8 Mon Sep 17 00:00:00 2001
From: Ayush3941 <ayushkgaur1 at gmail.com>
Date: Sun, 25 Jan 2026 22:00:57 -0500
Subject: [PATCH] [clang][bytecode] Avoid crash in constexpr wcslen on invalid
 argument type

---
 clang/lib/AST/ByteCode/InterpBuiltin.cpp | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/clang/lib/AST/ByteCode/InterpBuiltin.cpp b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
index d668fa118d3b9..e2517730e81ca 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltin.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
@@ -367,10 +367,18 @@ static bool interp__builtin_strlen(InterpState &S, CodePtr OpPC,
   unsigned ElemSize = StrPtr.getFieldDesc()->getElemSize();
 
   if (ID == Builtin::BI__builtin_wcslen || ID == Builtin::BIwcslen) {
-    [[maybe_unused]] const ASTContext &AC = S.getASTContext();
-    assert(ElemSize == AC.getTypeSizeInChars(AC.getWCharType()).getQuantity());
+    const ASTContext &AC = S.getASTContext();
+    unsigned WCharSize = AC.getTypeSizeInChars(AC.getWCharType()).getQuantity();
+    if (ElemSize != WCharSize) {
+      if (S.diagnosing()) {
+        //  a dedicated diagnostic
+        diagnoseNonConstexprBuiltin(S, OpPC, ID);
+      }
+      return false;
+    }
   }
 
+
   size_t Len = 0;
   for (size_t I = StrPtr.getIndex();; ++I, ++Len) {
     const Pointer &ElemPtr = StrPtr.atIndex(I);