[clang] [clang][bytecode] Diagnose reading mutable fields in Memcpy op (PR #197197)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/197197.diff


2 Files Affected:

- (modified) clang/lib/AST/ByteCode/InterpBuiltin.cpp (+6-4) 
- (modified) clang/test/AST/ByteCode/records.cpp (+10) 


``````````diff
diff --git a/clang/lib/AST/ByteCode/InterpBuiltin.cpp b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
index 5ba15b7ad4f63..726bae10b300d 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltin.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
@@ -6649,15 +6649,17 @@ static bool copyRecord(InterpState &S, CodePtr OpPC, const Pointer &Src,
   assert(SrcDesc->ElemRecord == DestDesc->ElemRecord);
   const Record *R = DestDesc->ElemRecord;
   for (const Record::Field &F : R->fields()) {
+    Pointer FP = Src.atField(F.Offset);
+
+    if (!CheckMutable(S, OpPC, FP))
+      return false;
+
     if (R->isUnion()) {
       // For unions, only copy the active field. Zero all others.
-      const Pointer &SrcField = Src.atField(F.Offset);
-      if (SrcField.isActive()) {
+      if (FP.isActive()) {
         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/test/AST/ByteCode/records.cpp b/clang/test/AST/ByteCode/records.cpp
index 20ead8cffdd99..b3f07c3cf007e 100644
--- a/clang/test/AST/ByteCode/records.cpp
+++ b/clang/test/AST/ByteCode/records.cpp
@@ -2011,3 +2011,13 @@ namespace RVOPtrIsExtern {
   }
   static_assert(test_all(), "");
 }
+
+namespace MutableInMemcpy {
+  union H {
+    mutable struct {} gx; // both-note {{declared here}}
+  };
+  constexpr H h1 = {};
+  constexpr H h2 = h1; // both-error {{must be initialized by a constant expression}} \
+                       // both-note {{read of mutable member 'gx' is not allowed in a constant expression}} \
+                       // both-note {{in call}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/197197