[clang][Sema] Propagate qualifiers during derived-to-base conversion

When accessing a field member through a derived-to-base conversion,
ensure qualifiers are propagated to the base class subobject.

Fixes: https://github.com/llvm/llvm-project/issues/127683.

clang/docs/ReleaseNotes.rst

@@ -340,6 +340,8 @@ Bug Fixes to C++ Support
 - Fixed an assertion failure affecting code that uses C++23 "deducing this". (#GH130272)
 - Clang now properly instantiates destructors for initialized members within non-delegating constructors. (#GH93251)
 - Correctly diagnoses if unresolved using declarations shadows template paramters (#GH129411)
+- Clang was previously coalescing volatile writes to members of volatile base class subobjects.
+  The issue has been addressed by propagating qualifiers during derived-to-base conversions in the AST. (#GH127824)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaExpr.cpp

@@ -3113,8 +3113,15 @@ Sema::PerformObjectMemberConversion(Expr *From,
                                    /*IgnoreAccess=*/true))
     return ExprError();
 
-  return ImpCastExprToType(From, DestType, CK_UncheckedDerivedToBase,
-                           VK, &BasePath);
+  // Propagate qualifiers to base subobjects as per:
+  // C++ [basic.type.qualifier]p1.2:
+  //   A volatile object is [...] a subobject of a volatile object.
+  Qualifiers FromTypeQuals = FromType.getQualifiers();
+  FromTypeQuals.setAddressSpace(DestType.getAddressSpace());
+  DestType = Context.getQualifiedType(DestType, FromTypeQuals);
+
+  return ImpCastExprToType(From, DestType, CK_UncheckedDerivedToBase, VK,
+                           &BasePath);
 }
 
 bool Sema::UseArgumentDependentLookup(const CXXScopeSpec &SS,

clang/test/CodeGenCXX/derived-to-base.cpp

@@ -46,4 +46,29 @@ namespace test3 {
   }
 }
 
+// Ensure volatile is preserved during derived-to-base conversion. 
+namespace PR127683 {
+
+struct Base {
+  int Val;
+};
+  
+struct Derived : Base { };
+  
+volatile Derived Obj;
+
+// CHECK-LABEL: define void @_ZN8PR12768319test_volatile_storeEv()
+// CHECK:         store volatile i32 0, ptr @_ZN8PR1276833ObjE, align 4
+void test_volatile_store() {
+  Obj.Val = 0;
+}
+
+// CHECK-LABEL: define void @_ZN8PR12768318test_volatile_loadEv()
+// CHECK:         %0 = load volatile i32, ptr @_ZN8PR1276833ObjE, align 4
+void test_volatile_load() {
+  [[maybe_unused]] int Val = Obj.Val;
+}
+
+}
+
 // CHECK: attributes [[NUW]] = { mustprogress noinline nounwind{{.*}} }

clang/test/SemaCXX/derived-to-base-propagate-qualifiers.cpp

@@ -0,0 +1,46 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -ast-dump -verify %s | FileCheck %s
+
+// Ensure qualifiers are preserved during derived-to-base conversion. 
+namespace PR127683 {
+
+struct Base {
+  int Val;
+};
+
+struct Derived : Base { };
+
+// Value-initialize base class subobjects with type qualifiers.
+volatile Derived VObj;
+const Derived CObj{}; // expected-note{{variable 'CObj' declared const here}}
+const volatile Derived CVObj{}; // expected-note{{variable 'CVObj' declared const here}}
+__attribute__((address_space(1))) Derived AddrSpaceObj{};
+
+void test_store() {
+  // CHECK: `-ImplicitCastExpr {{.*}} 'volatile PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  VObj.Val = 0;
+
+  // CHECK: `-ImplicitCastExpr {{.*}} 'const PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  CObj.Val = 1; // expected-error {{cannot assign to variable 'CObj' with const-qualified type 'const Derived'}}
+
+  // CHECK: `-ImplicitCastExpr {{.*}} 'const volatile PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  CVObj.Val = 1; // expected-error {{cannot assign to variable 'CVObj' with const-qualified type 'const volatile Derived'}}
+
+  // CHECK: `-ImplicitCastExpr {{.*}} '__attribute__((address_space(1))) PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  AddrSpaceObj.Val = 1;
+}
+
+void test_load() {
+  // CHECK: `-ImplicitCastExpr {{.*}} <col:30> 'volatile PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  [[maybe_unused]] int Val = VObj.Val;
+
+  // CHECK: `-ImplicitCastExpr {{.*}} 'const PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  Val = CObj.Val;
+  
+  // CHECK: `-ImplicitCastExpr {{.*}} 'const volatile PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  Val = CVObj.Val;
+
+  // CHECK: `-ImplicitCastExpr {{.*}} '__attribute__((address_space(1))) PR127683::Base' lvalue <UncheckedDerivedToBase (Base)>
+  Val = AddrSpaceObj.Val;
+}
+
+}