Generate valid C# for returned function pointers

Fixes https://github.com/mono/CppSharp/issues/1226.

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

src/AST/FunctionExtensions.cs

@@ -33,7 +33,9 @@ namespace CppSharp.AST
             {
                 @params.Add(new Parameter
                     {
-                        QualifiedType = universalDelegate && param.Kind == ParameterKind.IndirectReturnType ?
+                        QualifiedType = universalDelegate &&
+                            (param.Kind == ParameterKind.IndirectReturnType ||
+                             param.Type.Desugar().IsPointerTo(out FunctionType functionType)) ?
                             pointer : param.QualifiedType,
                         Kind = param.Kind,
                         Usage = param.Usage,

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -1835,7 +1835,7 @@ namespace CppSharp.Generators.CSharp
             }
 
             TypePrinterResult retType;
-            TypePrinter.PushMarshalKind(MarshalKind.GenericDelegate);
+            TypePrinter.PushMarshalKind(MarshalKind.VTableReturnValue);
             var @params = GatherInternalParams(method, out retType);
 
             var vTableMethodDelegateName = GetVTableMethodDelegateName(method);

src/Generator/Passes/DelegatesPass.cs

@@ -73,11 +73,13 @@ namespace CppSharp.Passes
                     ReturnType = method.ReturnType
                 };
 
+            TypePrinter.PushMarshalKind(MarshalKind.VTableReturnValue);
             functionType.Parameters.AddRange(
                 method.GatherInternalParams(Context.ParserOptions.IsItaniumLikeAbi, true));
 
             method.FunctionType = CheckForDelegate(new QualifiedType(functionType),
                 method.Namespace, @private: true);
+            TypePrinter.PopMarshalKind();
 
             return true;
         }

tests/Common/Common.Tests.cs

@@ -576,6 +576,9 @@ public class CommonTests : GeneratorTestFixture
             Assert.That(prop.conflict, Is.EqualTo(CommonTest.TestProperties.Conflict.Value1));
             prop.conflict = CommonTest.TestProperties.Conflict.Value2;
             Assert.That(prop.conflict, Is.EqualTo(CommonTest.TestProperties.Conflict.Value2));
+
+            prop.Callback = x => 4 * x;
+            Assert.That(prop.Callback(5), Is.EqualTo(20));
         }
         using (var prop = new HasOverridenSetter())
         {

tests/Common/Common.cpp

@@ -718,6 +718,16 @@ void TestProperties::SetConflict(Conflict conflict)
     _conflict = conflict;
 }
 
+int(*TestProperties::getCallback())(int)
+{
+    return _callback;
+}
+
+void TestProperties::setCallback(int(*value)(int))
+{
+    _callback = value;
+}
+
 HasOverridenSetter::HasOverridenSetter()
 {
 }

tests/Common/Common.h

@@ -649,6 +649,9 @@ public:
     Conflict GetConflict();
     void SetConflict(Conflict _conflict);
 
+    virtual int(*getCallback())(int);
+    virtual void setCallback(int(*value)(int));
+
 private:
     int FieldValue;
     double _refToPrimitiveInSetter;
@@ -656,6 +659,7 @@ private:
     int _setterReturnsBoolean;
     int _virtualSetterReturnsBoolean;
     Conflict _conflict;
+    int(*_callback)(int);
 };
 
 class DLL_API HasOverridenSetter : public TestProperties