fix: initialize nullable members for nested containers

src/Riok.Mapperly/Descriptors/MappingBodyBuilders/BuilderContext/MembersContainerBuilderContext.cs

@@ -15,7 +15,7 @@ public class MembersContainerBuilderContext<T>(MappingBuilderContext builderCont
     where T : IMemberAssignmentTypeMapping
 {
     private readonly Dictionary<MemberPath, MemberNullDelegateAssignmentMapping> _nullDelegateMappings = new();
-    private readonly HashSet<MemberPath> _initializedNullableTargetPaths = [];
+    private readonly HashSet<(IMemberAssignmentMappingContainer, MemberPath)> _initializedNullableTargetPaths = [];
 
     public void AddMemberAssignmentMapping(IMemberAssignmentMapping memberMapping) => AddMemberAssignmentMapping(Mapping, memberMapping);
 
@@ -79,7 +79,7 @@ private void AddMemberAssignmentMapping(IMemberAssignmentMappingContainer contai
         // the target should be non-null after this assignment and can be set as initialized.
         if (!mapping.MemberInfo.IsSourceNullable && mapping.MemberInfo.TargetMember.MemberType.IsNullable())
         {
-            _initializedNullableTargetPaths.Add(mapping.MemberInfo.TargetMember);
+            _initializedNullableTargetPaths.Add((container, mapping.MemberInfo.TargetMember));
         }
     }
 
@@ -93,7 +93,10 @@ private void AddNullMemberInitializers(IMemberAssignmentMappingContainer contain
             if (!nullablePath.Member.CanSet)
                 continue;
 
-            if (!_initializedNullableTargetPaths.Add(nullablePath))
+            if (_initializedNullableTargetPaths.Contains((Mapping, nullablePath)))
+                continue;
+
+            if (!_initializedNullableTargetPaths.Add((container, nullablePath)))
                 continue;
 
             if (!BuilderContext.InstanceConstructors.TryBuild(BuilderContext.Source, type, out var ctor))

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

@@ -588,6 +588,45 @@ public partial B Map(A a)
             );
     }
 
+    [Fact]
+    public void NullableNestedMembersShouldInitializeWithNoNullAssignment()
+    {
+        var source = TestSourceBuilder.MapperWithBodyAndTypes(
+            """
+            [MapProperty("NullableValue1", "V.NullableValue1")]
+            [MapProperty("NullableValue2", "V.NullableValue2")]
+            public partial B Map(A a)
+            """,
+            TestSourceBuilderOptions.Default with
+            {
+                AllowNullPropertyAssignment = false,
+            },
+            "class A { public int? NullableValue1 { get; set; } public int? NullableValue2 { get; set; } }",
+            "class B { public C? V { get; set; } }",
+            "class C { public int? NullableValue1 { get; set; } public int? NullableValue2 { get; set; } }"
+        );
+
+        TestHelper
+            .GenerateMapper(source)
+            .Should()
+            .HaveMapMethodBody(
+                """
+                var target = new global::B();
+                if (a.NullableValue1 != null)
+                {
+                    target.V ??= new global::C();
+                    target.V.NullableValue1 = a.NullableValue1.Value;
+                }
+                if (a.NullableValue2 != null)
+                {
+                    target.V ??= new global::C();
+                    target.V.NullableValue2 = a.NullableValue2.Value;
+                }
+                return target;
+                """
+            );
+    }
+
     [Fact]
     public void NullableClassToNullableClassPropertyThrowShouldSetNull()
     {