[clang][bytecode] Refuse to contruct objects with virtual bases (#110142)

Author: tbaederr

clang/lib/AST/ByteCode/Interp.cpp

@@ -1043,6 +1043,25 @@ bool CheckLiteralType(InterpState &S, CodePtr OpPC, const Type *T) {
   return false;
 }
 
+static bool checkConstructor(InterpState &S, CodePtr OpPC, const Function *Func,
+                             const Pointer &ThisPtr) {
+  assert(Func->isConstructor());
+
+  const Descriptor *D = ThisPtr.getFieldDesc();
+
+  // FIXME: I think this case is not 100% correct. E.g. a pointer into a
+  // subobject of a composite array.
+  if (!D->ElemRecord)
+    return true;
+
+  if (D->ElemRecord->getNumVirtualBases() == 0)
+    return true;
+
+  S.FFDiag(S.Current->getLocation(OpPC), diag::note_constexpr_virtual_base)
+      << Func->getParentDecl();
+  return false;
+}
+
 bool CallVar(InterpState &S, CodePtr OpPC, const Function *Func,
              uint32_t VarArgSize) {
   if (Func->hasThisPointer()) {
@@ -1117,6 +1136,9 @@ bool Call(InterpState &S, CodePtr OpPC, const Function *Func,
       if (!CheckInvoke(S, OpPC, ThisPtr))
         return cleanup();
     }
+
+    if (Func->isConstructor() && !checkConstructor(S, OpPC, Func, ThisPtr))
+      return false;
   }
 
   if (!CheckCallable(S, OpPC, Func))

clang/test/AST/ByteCode/cxx23.cpp

@@ -158,6 +158,21 @@ namespace VirtualBases {
     /// Calls the constructor of D.
     D d;
   }
+
+#if __cplusplus >= 202302L
+  struct VBase {};
+  struct HasVBase : virtual VBase {}; // all23-note 1{{virtual base class declared here}}
+  struct Derived : HasVBase {
+    constexpr Derived() {} // all23-error {{constexpr constructor not allowed in struct with virtual base class}}
+  };
+  template<typename T> struct DerivedFromVBase : T {
+    constexpr DerivedFromVBase();
+  };
+  constexpr int f(DerivedFromVBase<HasVBase>) {}
+  template<typename T> constexpr DerivedFromVBase<T>::DerivedFromVBase() : T() {}
+  constexpr int nVBase = (DerivedFromVBase<HasVBase>(), 0); // all23-error {{constant expression}} \
+                                                            // all23-note {{cannot construct object of type 'DerivedFromVBase<VirtualBases::HasVBase>' with virtual base class in a constant expression}}
+#endif
 }
 
 namespace LabelGoto {