Fixed the mapping of std::string not to destroy values when a reference (&) is returned.

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

src/Generator/Types/Std/Stdlib.cs

@@ -92,13 +92,13 @@ namespace CppSharp.Types.Std
 
         public override void CSharpMarshalToManaged(CSharpMarshalContext ctx)
         {
-            var type = ctx.ReturnType.Type;
+            var type = ctx.ReturnType.Type.Desugar();
             ClassTemplateSpecialization basicString = GetBasicString(type);
             Declaration c_str = basicString.Methods.FirstOrDefault(m => m.OriginalName == "c_str");
             if (!c_str.IsGenerated)
                 c_str = basicString.Properties.First(p => p.OriginalName == "c_str");
             var typePrinter = new CSharpTypePrinter(ctx.Context);
-            if (type.IsPointer() || ctx.Declaration is Field)
+            if (type.IsAddress() || ctx.Declaration is Field)
             {
                 ctx.Return.Write("{0}.{1}({2}).{3}{4}",
                     basicString.Visit(typePrinter), Helpers.CreateInstanceIdentifier,

tests/Common/Common.Tests.cs

@@ -665,6 +665,8 @@ This is a very long string. This is a very long string. This is a very long stri
             Assert.That(hasStdString.testStdString(t), Is.EqualTo(t + "_test"));
             hasStdString.s = t;
             Assert.That(hasStdString.s, Is.EqualTo(t));
+            Assert.That(hasStdString.stdString, Is.EqualTo(t));
+            Assert.That(hasStdString.stdString, Is.EqualTo(t));
         }
     }
 

tests/Common/Common.cpp

@@ -430,6 +430,11 @@ std::string HasStdString::testStdString(std::string s)
     return s + "_test";
 }
 
+std::string& HasStdString::getStdString()
+{
+    return s;
+}
+
 TypeMappedIndex::TypeMappedIndex()
 {
 }

tests/Common/Common.h

@@ -759,6 +759,7 @@ class DLL_API HasStdString
 public:
     std::string testStdString(std::string s);
     std::string s;
+    std::string& getStdString();
 };
 
 class DLL_API InternalCtorAmbiguity