Fix PInvokeStackImbalance when calling C++ methods (#1453)

* Fix PInvokeStackImbalance when calling C++ methods

src/Generator/Passes/CheckAbiParameters.cs

@@ -1,4 +1,5 @@
 using CppSharp.AST;
+using CppSharp.AST.Extensions;
 using System.Linq;
 
 namespace CppSharp.Passes
@@ -29,7 +30,15 @@ namespace CppSharp.Passes
             if (!VisitDeclaration(function))
                 return false;
 
-            if (function.IsReturnIndirect)
+            var isReturnIndirect = function.IsReturnIndirect || (
+                    Context.ParserOptions.IsMicrosoftAbi &&
+                    function is Method &&
+                    !function.ReturnType.Type.Desugar().IsAddress()  &&
+                    function.ReturnType.Type.Desugar().TryGetDeclaration(out Class returnTypeDecl) && 
+                    returnTypeDecl.IsPOD && 
+                    returnTypeDecl.Layout.Size <= 8);
+
+            if (isReturnIndirect)
             {
                 var indirectParam = new Parameter()
                     {

tests/CSharp/CSharp.Tests.cs

@@ -159,6 +159,19 @@ public unsafe class CSharpTests : GeneratorTestFixture
         }
     }
 
+    [Test]
+    public void TestReturnSmallPOD()
+    {
+        using (var f = new Foo())
+        {
+            foreach(var pod in new[] { f.SmallPodCdecl, f.SmallPodStdcall, f.SmallPodFastcall, f.SmallPodThiscall, f.SmallPodVectorcall })
+            {
+                Assert.That(pod.A, Is.EqualTo(10000));
+                Assert.That(pod.B, Is.EqualTo(40000));
+            }
+        }
+    }
+
     [Test]
     public void TestPropertyAccessModifier()
     {

tests/CSharp/CSharp.cpp

@@ -130,7 +130,6 @@ const Foo& Bar::operator[](int i) const
     return m_foo;
 }
 
-
 Quux::Quux() : _setterWithDefaultOverload(0)
 {
 

tests/CSharp/CSharp.h

@@ -9,6 +9,12 @@
 #include "ExcludedUnit.hpp"
 #include "CSharpTemplates.h"
 
+struct SmallPOD
+{
+    int a;
+    int b;
+};
+
 class DLL_API Foo
 {
 public:
@@ -40,6 +46,12 @@ public:
     static int propertyCall();
     static int getGetPropertyCall();
 
+    SmallPOD CDECL getSmallPod_cdecl() { return { 10000, 40000 }; }
+    SmallPOD STDCALL getSmallPod_stdcall() { return { 10000, 40000 }; }
+    SmallPOD FASTCALL getSmallPod_fastcall() { return { 10000, 40000 }; }
+    SmallPOD THISCALL getSmallPod_thiscall() { return { 10000, 40000 }; }
+    SmallPOD VECTORCALL getSmallPod_vectorcall() { return { 10000, 40000 }; }
+
     int operator ++();
     int operator --();
     operator const char*() const;

tests/Tests.h

@@ -15,6 +15,19 @@
 #ifndef CDECL
 #define CDECL __cdecl
 #endif
+
+#ifndef FASTCALL
+#define FASTCALL __fastcall
+#endif
+
+#ifndef THISCALL
+#define THISCALL __thiscall
+#endif
+
+#ifndef VECTORCALL
+#define VECTORCALL __vectorcall
+#endif
+
 #else
 #define DLL_API __attribute__ ((visibility ("default")))
 
@@ -30,6 +43,21 @@
 #ifndef CDECL
 #define CDECL __attribute__((cdecl))
 #endif
+
+#ifndef FASTCALL
+#define FASTCALL
+#endif
+
+#ifndef THISCALL
+#define THISCALL
+#endif
+
+#ifndef VECTORCALL
+#define VECTORCALL
+#endif
+
+#define 
+
 #endif
 
 #define CS_OUT