[clang] [clang][bytecode] Fix crash when dereferencing cast to larger type (PR #179030)

[via cfe-commits]

https://github.com/mugiwaraluffy56 updated https://github.com/llvm/llvm-project/pull/179030

>From 23f63073d6253b96f8e53ecc83c232d1e3b1dab0 Mon Sep 17 00:00:00 2001
From: mugiwaraluffy56 <myakampuneeth at gmail.com>
Date: Sat, 31 Jan 2026 18:02:24 +0530
Subject: [PATCH] [clang][ByteCode] Fix crash when dereferencing cast to larger
 type

When dereferencing a pointer that was reinterpret_cast to a larger type
(e.g. *(int**)""), check if the pointer descriptor's primitive type
matches what we're trying to read. If not, return std::nullopt to
gracefully fail the constant evaluation instead of crashing.

Fixes #179015
---
 clang/lib/AST/ByteCode/Pointer.cpp  | 3 +++
 clang/test/AST/ByteCode/invalid.cpp | 6 ++++++
 2 files changed, 9 insertions(+)

diff --git a/clang/lib/AST/ByteCode/Pointer.cpp b/clang/lib/AST/ByteCode/Pointer.cpp
index a1ab492e5cb37..b625128514f83 100644
--- a/clang/lib/AST/ByteCode/Pointer.cpp
+++ b/clang/lib/AST/ByteCode/Pointer.cpp
@@ -947,6 +947,9 @@ std::optional<APValue> Pointer::toRValue(const Context &Ctx,
 
   // Just load primitive types.
   if (OptPrimType T = Ctx.classify(ResultType)) {
+    if (const Descriptor *D = getFieldDesc();
+        (D->isPrimitive() || D->isPrimitiveArray()) && D->getPrimType() != *T)
+      return std::nullopt;
     TYPE_SWITCH(*T, return this->deref<T>().toAPValue(ASTCtx));
   }
 
diff --git a/clang/test/AST/ByteCode/invalid.cpp b/clang/test/AST/ByteCode/invalid.cpp
index f7f2da2769d65..7c9433ec065ee 100644
--- a/clang/test/AST/ByteCode/invalid.cpp
+++ b/clang/test/AST/ByteCode/invalid.cpp
@@ -57,6 +57,12 @@ namespace Casts {
 
   /// Just make sure this doesn't crash.
   float PR9558 = reinterpret_cast<const float&>("asd");
+
+  /// Ensure we don't crash when trying to dereference a cast pointer where the
+  /// target type is larger than the source allocation (GH#179015).
+  void GH179015() {
+    *(int **)""; // both-warning {{expression result unused}}
+  }
 }