Delete the C++ copy when returning by value (#1623)

Fixes https://github.com/mono/CppSharp/issues/1600.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

src/Generator/Generators/CSharp/CSharpMarshal.cs

@@ -282,7 +282,48 @@ namespace CppSharp.Generators.CSharp
             if (returnType.IsAddress())
                 Context.Return.Write(HandleReturnedPointer(@class, qualifiedClass));
             else
+            {
+                if (Context.MarshalKind == MarshalKind.NativeField ||
+                    Context.MarshalKind == MarshalKind.Variable ||
+                    !originalClass.HasNonTrivialDestructor)
+                {
                     Context.Return.Write($"{qualifiedClass}.{Helpers.CreateInstanceIdentifier}({Context.ReturnVarName})");
+                }
+                else
+                {
+                    Context.Before.WriteLine($@"var __{Context.ReturnVarName} = {
+                        qualifiedClass}.{Helpers.CreateInstanceIdentifier}({Context.ReturnVarName});");
+                    Method dtor = originalClass.Destructors.First();
+                    if (dtor.IsVirtual)
+                    {
+                        var i = VTables.GetVTableIndex(dtor);
+                        int vtableIndex = 0;
+                        if (Context.Context.ParserOptions.IsMicrosoftAbi)
+                            vtableIndex = @class.Layout.VFTables.IndexOf(@class.Layout.VFTables.First(
+                                v => v.Layout.Components.Any(c => c.Method == dtor)));
+                        Context.Before.WriteLine($@"var __vtables = new IntPtr[] {{ {
+                            string.Join(", ", originalClass.Layout.VTablePointers.Select(
+                                x => $" * (IntPtr*) ({ Helpers.InstanceIdentifier} + {x.Offset})"))} }};");
+                        Context.Before.WriteLine($"var __slot = *(IntPtr*) (__vtables[{vtableIndex}] + {i} * sizeof(IntPtr));");
+                        Context.Before.Write($"Marshal.GetDelegateForFunctionPointer<{dtor.FunctionType}>(__slot)({Helpers.InstanceIdentifier}");
+                        if (dtor.GatherInternalParams(Context.Context.ParserOptions.IsItaniumLikeAbi).Count > 1)
+                        {
+                            Context.Before.WriteLine(", 0");
+                        }
+                        Context.Before.WriteLine(");");
+                    }
+                    else
+                    {
+                        string suffix = string.Empty;
+                        var specialization = @class as ClassTemplateSpecialization;
+                        if (specialization != null)
+                            suffix = Helpers.GetSuffixFor(specialization);
+                        Context.Before.WriteLine($@"{typePrinter.PrintNative(originalClass)}.dtor{
+                            suffix}(new {typePrinter.IntPtrType}(&{Context.ReturnVarName}));");
+                    }
+                    Context.Return.Write($"__{Context.ReturnVarName}");
+                }
+            }
 
             return true;
         }

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -1334,8 +1334,15 @@ namespace CppSharp.Generators.CSharp
                 AddBlock(new Block(BlockKind.Unreachable));
                 return;
             }
-            GenerateFunctionInProperty(@class, actualProperty.GetMethod, actualProperty,
-                property.QualifiedType);
+            QualifiedType type = default;
+            if (actualProperty != property ||
+                // indexers
+                (property.QualifiedType.Type.IsPrimitiveType() && 
+                 actualProperty.GetMethod.ReturnType.Type.IsPointerToPrimitiveType()))
+            {
+                type = property.QualifiedType;
+            }
+            GenerateFunctionInProperty(@class, actualProperty.GetMethod, actualProperty, type);
         }
 
         private static Property GetActualProperty(Property property, Class c)
@@ -2180,10 +2187,8 @@ namespace CppSharp.Generators.CSharp
 
                 // ensure any virtual dtor in the chain is called
                 var dtor = @class.Destructors.FirstOrDefault(d => d.Access != AccessSpecifier.Private);
-                var baseDtor = @class.BaseClass == null ? null :
-                    @class.BaseClass.Destructors.FirstOrDefault(d => !d.IsVirtual);
                 if (ShouldGenerateClassNativeField(@class) ||
-                    ((dtor != null && (dtor.IsVirtual || @class.HasNonTrivialDestructor)) && baseDtor != null) ||
+                    (dtor != null && (dtor.IsVirtual || @class.HasNonTrivialDestructor)) ||
                     // virtual destructors in abstract classes may lack a pointer in the v-table
                     // so they have to be called by symbol; thus we need an explicit Dispose override
                     @class.IsAbstract)
@@ -3464,11 +3469,6 @@ internal static{(@new ? " new" : string.Empty)} {printedClass} __GetInstance({Ty
             if (function.IsPure)
                 return;
 
-            if (function.OriginalName == "system_do_it")
-            {
-                System.Diagnostics.Debugger.Break();
-            }
-
             PushBlock(BlockKind.InternalsClassMethod);
             var callConv = function.CallingConvention.ToInteropCallConv();
 

tests/CSharp/CSharpTemplates.h

@@ -794,6 +794,7 @@ template class DLL_API Base<int>;
 template class DLL_API DependentValueFields<int>;
 template class DLL_API DependentValueFields<int*>;
 template class DLL_API DependentValueFields<float>;
+template class DLL_API DependentValueFields<double>;
 template class DLL_API DependentPointerFields<float>;
 template class DLL_API VirtualTemplate<int>;
 template class DLL_API VirtualTemplate<bool>;

tests/Common/Common.Tests.cs

@@ -1127,6 +1127,15 @@ This is a very long string. This is a very long string. This is a very long stri
         }
     }
 
+    [Test]
+    public void TestReturnByValueWithReturnParam()
+    {
+        using (var returnByValueWithReturnParam = ReturnByValueWithReturnParamFactory.Generate())
+        {
+            Assert.That(returnByValueWithReturnParam.UseCount, Is.EqualTo(1));
+        }
+    }
+
     [Test, Ignore("This was exposed by another bug and doesn't work yet.")]
     public void TestFreeFunctionReturnByValue()
     {

tests/Common/Common.cpp

@@ -1336,6 +1336,19 @@ HasPropertyNamedAsParent::~HasPropertyNamedAsParent()
 {
 }
 
+ReturnByValueWithReturnParam::ReturnByValueWithReturnParam() {}
+
+ReturnByValueWithReturnParam::ReturnByValueWithReturnParam(const ReturnByValueWithReturnParam& other) : _ptr(other._ptr) {}
+
+ReturnByValueWithReturnParam::~ReturnByValueWithReturnParam() {}
+
+int ReturnByValueWithReturnParam::getUseCount() { return _ptr.use_count(); }
+
+ReturnByValueWithReturnParam ReturnByValueWithReturnParamFactory::generate()
+{
+    return ReturnByValueWithReturnParam();
+}
+
 void integerOverload(int i)
 {
 }

tests/Common/Common.h

@@ -1355,6 +1355,24 @@ public:
     int hasPropertyNamedAsParent;
 };
 
+class DLL_API ReturnByValueWithReturnParam
+{
+public:
+    ReturnByValueWithReturnParam();
+    ReturnByValueWithReturnParam(const ReturnByValueWithReturnParam& other);
+    ~ReturnByValueWithReturnParam();
+    int getUseCount();
+
+private:
+    std::shared_ptr<int> _ptr = std::shared_ptr<int>(new int[1]);
+};
+
+class DLL_API ReturnByValueWithReturnParamFactory
+{
+public:
+    static ReturnByValueWithReturnParam generate();
+};
+
 template<typename T> void TemplatedFunction(T type)
 {