fix: prevent mapping backing field (#608)

src/Riok.Mapperly/Descriptors/SymbolAccessor.cs

@@ -235,8 +235,9 @@ private IEnumerable<IMappableMember> GetAllAccessibleMappableMembersCore(ITypeSy
             return namedType.TupleElements.Select(x => MappableMember.Create(this, x)).WhereNotNull();
         }
 
+        // member must be property or a none backing variable field
         return GetAllMembers(symbol)
-            .Where(x => x is { IsStatic: false, Kind: SymbolKind.Property or SymbolKind.Field })
+            .Where(x => x is { IsStatic: false, Kind: SymbolKind.Property } or IFieldSymbol { IsStatic: false, AssociatedSymbol: null })
             .DistinctBy(x => x.Name)
             .Select(x => MappableMember.Create(this, x))
             .WhereNotNull();

test/Riok.Mapperly.Tests/Mapping/ObjectVisibilityTest.cs

@@ -212,4 +212,25 @@ internal partial class Mapper
 
         return TestHelper.VerifyGenerator(source);
     }
+
+    [Fact]
+    public Task PropertyInTheSameClassShouldIgnoreBackingField()
+    {
+        var source = TestSourceBuilder.CSharp(
+            """
+            public partial class A
+            {
+                public string Value { get; set; }
+
+                [Riok.Mapperly.Abstractions.Mapper(UseDeepCloning = true)]
+                internal partial class Mapper
+                {
+                    public partial A Clone(A source);
+                }
+            }
+            """
+        );
+
+        return TestHelper.VerifyGenerator(source);
+    }
 }

test/Riok.Mapperly.Tests/_snapshots/ObjectVisibilityTest.PropertyInTheSameClassShouldIgnoreBackingField#A.Mapper.g.verified.cs

@@ -0,0 +1,15 @@
+//HintName: A.Mapper.g.cs
+// <auto-generated />
+#nullable enable
+public partial class A
+{
+    internal partial class Mapper
+    {
+        public partial global::A Clone(global::A source)
+        {
+            var target = new global::A();
+            target.Value = source.Value;
+            return target;
+        }
+    }
+}