[clang][bytecode] Fix const-in-mutable fields

For mutable and const fields, we have two bits in InlineDescriptor, which both get inherited down the hierarchy. When a field is both const and mutable, we CAN read from it if it is a mutable-in-const field, but we _can't_ read from it if it is a const-in-mutable field. We need another bit to distinguish the two cases.
llvm · tbaederr · Jul 18, 2025 · Jul 17, 2025 · Jul 17, 2025 · Jul 17, 2025
commit 34201c4031f90a09f4e4d8295f7fb4a261bbb864
diff --git a/clang/lib/AST/ByteCode/Descriptor.cpp b/clang/lib/AST/ByteCode/Descriptor.cpp
@@ -162,6 +162,8 @@ static void initField(Block *B, std::byte *Ptr, bool IsConst, bool IsMutable,
   Desc->IsConst = IsConst || D->IsConst;
   Desc->IsFieldMutable = IsMutable || D->IsMutable;
   Desc->IsVolatile = IsVolatile || D->IsVolatile;
+  // True if this field is const AND the parent is mutable.
+  Desc->IsConstInMutable = Desc->IsConst && IsMutable;
 
   if (auto Fn = D->CtorFn)
     Fn(B, Ptr + FieldOffset, Desc->IsConst, Desc->IsFieldMutable,

diff --git a/clang/lib/AST/ByteCode/Descriptor.h b/clang/lib/AST/ByteCode/Descriptor.h
@@ -101,6 +101,10 @@ struct InlineDescriptor {
   /// Flag indicating if the field is mutable (if in a record).
   LLVM_PREFERRED_TYPE(bool)
   unsigned IsFieldMutable : 1;
+  /// Flag indicating if this field is a const field nested in
+  /// a mutable parent field.
+  LLVM_PREFERRED_TYPE(bool)
+  unsigned IsConstInMutable : 1;
   /// Flag indicating if the field is an element of a composite array.
   LLVM_PREFERRED_TYPE(bool)
   unsigned IsArrayElement : 1;

diff --git a/clang/lib/AST/ByteCode/Disasm.cpp b/clang/lib/AST/ByteCode/Disasm.cpp
@@ -445,6 +445,7 @@ LLVM_DUMP_METHOD void InlineDescriptor::dump(llvm::raw_ostream &OS) const {
   OS << "InUnion: " << InUnion << "\n";
   OS << "IsFieldMutable: " << IsFieldMutable << "\n";
   OS << "IsArrayElement: " << IsArrayElement << "\n";
+  OS << "IsConstInMutable: " << IsConstInMutable << '\n';
   OS << "Desc: ";
   if (Desc)
     Desc->dump(OS);

diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
@@ -566,7 +566,10 @@ bool CheckDowncast(InterpState &S, CodePtr OpPC, const Pointer &Ptr,
 
 bool CheckConst(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   assert(Ptr.isLive() && "Pointer is not live");
-  if (!Ptr.isConst() || Ptr.isMutable())
+  if (!Ptr.isConst())
+    return true;
+
+  if (Ptr.isMutable() && !Ptr.isConstInMutable())
     return true;
 
   if (!Ptr.isBlockPointer())

diff --git a/clang/lib/AST/ByteCode/Pointer.h b/clang/lib/AST/ByteCode/Pointer.h
@@ -576,6 +576,11 @@ class Pointer {
       return true;
     return isRoot() ? getDeclDesc()->IsConst : getInlineDesc()->IsConst;
   }
+  bool isConstInMutable() const {
+    if (!isBlockPointer())
+      return false;
+    return isRoot() ? false : getInlineDesc()->IsConstInMutable;
+  }
 
   /// Checks if an object or a subfield is volatile.
   bool isVolatile() const {

diff --git a/clang/test/AST/ByteCode/mutable.cpp b/clang/test/AST/ByteCode/mutable.cpp
@@ -1,11 +1,7 @@
-// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -std=c++11 -verify=expected,expected11,both,both11 %s
-// RUN: %clang_cc1 -fexperimental-new-constant-interpreter -std=c++14 -verify=expected,expected14,both %s
-// RUN: %clang_cc1 -std=c++11 -verify=ref,ref11,both,both11 %s
-// RUN: %clang_cc1 -std=c++14 -verify=ref,ref14,both %s
-
-
-
-
+// RUN: %clang_cc1 -std=c++11 -verify=expected,expected11,both,both11 %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++14 -verify=expected,expected14,both        %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -std=c++11 -verify=ref,ref11,both,both11           %s
+// RUN: %clang_cc1 -std=c++14 -verify=ref,ref14,both                  %s
 
 namespace Simple {
   struct S {
@@ -26,3 +22,47 @@ namespace Simple {
   static_assert(s2.a2 == 12, ""); // both11-error {{not an integral constant expression}} \
                                   // both11-note {{initializer of 's2' is not a constant expression}}
 }
+#if __cplusplus >= 201402L
+namespace ConstInMutable {
+  class B {
+    public:
+
+    const int f;
+    constexpr B() : f(12) {}
+  };
+  class A {
+    public:
+    mutable B b;
+    constexpr A() = default;
+  };
+  constexpr int constInMutable() {
+    A a;
+
+    int *m = (int*)&a.b.f;
+    *m = 12; // both-note {{modification of object of const-qualified type 'const int' is not allowed in a constant expression}}
+    return 1;
+  }
+  static_assert(constInMutable() == 1, ""); // both-error {{not an integral constant expression}} \
+                                            // both-note {{in call to}}
+}
+
+namespace MutableInConst {
+  class C {
+  public:
+    mutable int c;
+    constexpr C() : c(50) {}
+  };
+  class D {
+  public:
+    C c;
+    constexpr D() {}
+  };
+  constexpr int mutableInConst() {
+    const D d{};
+    int *m = (int*)&d.c.c;
+    *m = 12;
+    return 1;
+  }
+  static_assert(mutableInConst() == 1, "");
+}
+#endif