[clang] [clang][bytecode] Report mutable reads when copying unions (PR #149320)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/149320

None

>From 896c606f240e302f12a891aa9ddabba70f2e6364 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Thu, 17 Jul 2025 16:28:15 +0200
Subject: [PATCH] [clang][bytecode] Report mutable reads when copying unions

---
 clang/lib/AST/ByteCode/InterpBuiltin.cpp |  2 ++
 clang/lib/AST/ByteCode/InterpFrame.cpp   |  5 +++++
 clang/test/AST/ByteCode/unions.cpp       | 14 ++++++++++++++
 3 files changed, 21 insertions(+)

diff --git a/clang/lib/AST/ByteCode/InterpBuiltin.cpp b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
index de0b97fd93c76..ace7016785069 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltin.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
@@ -2906,6 +2906,8 @@ static bool copyRecord(InterpState &S, CodePtr OpPC, const Pointer &Src,
         if (!copyField(F, /*Activate=*/true))
           return false;
       } else {
+        if (!CheckMutable(S, OpPC, Src.atField(F.Offset)))
+          return false;
         Pointer DestField = Dest.atField(F.Offset);
         zeroAll(DestField);
       }
diff --git a/clang/lib/AST/ByteCode/InterpFrame.cpp b/clang/lib/AST/ByteCode/InterpFrame.cpp
index a5a4bd25fe712..d62a4f6275b50 100644
--- a/clang/lib/AST/ByteCode/InterpFrame.cpp
+++ b/clang/lib/AST/ByteCode/InterpFrame.cpp
@@ -128,6 +128,11 @@ static bool shouldSkipInBacktrace(const Function *F) {
   if (FD->getDeclName().getCXXOverloadedOperator() == OO_New ||
       FD->getDeclName().getCXXOverloadedOperator() == OO_Array_New)
     return true;
+
+  if (const auto *MD = dyn_cast<CXXMethodDecl>(FD);
+      MD && MD->getParent()->isAnonymousStructOrUnion())
+    return true;
+
   return false;
 }
 
diff --git a/clang/test/AST/ByteCode/unions.cpp b/clang/test/AST/ByteCode/unions.cpp
index 0fa44a259a4ff..7cfd0d677a7b3 100644
--- a/clang/test/AST/ByteCode/unions.cpp
+++ b/clang/test/AST/ByteCode/unions.cpp
@@ -847,6 +847,20 @@ namespace Activation2 {
   }
   static_assert(change_member_indirectly() == 4);
 }
+
+namespace CopyCtorMutable {
+  struct E {
+    union { // expected-note {{read of mutable member 'b'}}
+      int a;
+      mutable int b; // both-note {{here}}
+    };
+  };
+  constexpr E e1 = {{1}};
+  constexpr E e2 = e1; // both-error {{constant}} \
+                       // ref-note {{read of mutable member 'b'}} \
+                       // both-note {{in call}}
+}
+
 #endif
 
 namespace AddressComparison {