Null source member should overwrite existing destination member

src/AutoMapper/Execution/ExpressionBuilder.cs

@@ -11,6 +11,7 @@ public static class ExpressionBuilder
     public static readonly Expression True = Constant(true, typeof(bool));
     public static readonly Expression Null = Constant(null, typeof(object));
     public static readonly Expression Empty = Empty();
+    public static readonly LambdaExpression EmptyLambda = Expression.Lambda(Empty);
     public static readonly Expression Zero = Constant(0, typeof(int));
     public static readonly ParameterExpression ExceptionParameter = Parameter(typeof(Exception), "ex");
     public static readonly ParameterExpression ContextParameter = Parameter(typeof(ResolutionContext), "context");
@@ -74,7 +75,8 @@ public static Expression MapExpression(this IGlobalConfiguration configuration,
         if (typeMap != null)
         {
             var allowNull = memberMap?.AllowNull;
-            nullCheck = !typeMap.HasTypeConverter && allowNull.HasValue && allowNull != profileMap.AllowNullDestinationValues;
+            nullCheck = !typeMap.HasTypeConverter && (destination.NodeType != ExpressionType.Default ||
+                (allowNull.HasValue && allowNull != profileMap.AllowNullDestinationValues));
             if (!typeMap.HasDerivedTypesToInclude)
             {
                 typeMap.Seal(configuration);

src/AutoMapper/Execution/TypeMapPlanBuilder.cs

@@ -288,7 +288,7 @@ private Expression CreateConstructorParameterExpression(ConstructorParameterMap
         var customSource = GetCustomSource(ctorParamMap);
         var resolvedExpression = BuildValueResolverFunc(ctorParamMap, customSource, defaultValue);
         var resolvedValue = Variable(resolvedExpression.Type, "resolvedValue");
-        var mapMember = MapMember(ctorParamMap, defaultValue, resolvedValue);
+        var mapMember = MapMember(ctorParamMap, resolvedValue, defaultValue);
         _variables.Clear();
         _variables.Add(resolvedValue);
         _expressions.Clear();
@@ -328,7 +328,7 @@ private Expression CreatePropertyMapFunc(MemberMap memberMap, Expression destina
         var valueResolver = BuildValueResolverFunc(memberMap, customSource, destinationMemberGetter);
         var resolvedValueVariable = Variable(valueResolver.Type, "resolvedValue");
         var destinationMemberValue = DestinationMemberValue(memberMap, destinationMemberGetter, destinationMemberReadOnly);
-        var mappedMember = MapMember(memberMap, destinationMemberValue, resolvedValueVariable);
+        var mappedMember = MapMember(memberMap, resolvedValueVariable, destinationMemberValue);
         var mappedMemberVariable = SetVariables(valueResolver, resolvedValueVariable, mappedMember);
         var mapperExpr = destinationMemberReadOnly ? (Expression)mappedMemberVariable : Assign(destinationMemberAccess, mappedMemberVariable);
         if (memberMap.Condition != null)
@@ -382,7 +382,7 @@ ParameterExpression SetVariables(Expression valueResolver, ParameterExpression r
         }
         return mappedMemberVariable;
     }
-    Expression MapMember(MemberMap memberMap, Expression destinationMemberValue, ParameterExpression resolvedValue)
+    Expression MapMember(MemberMap memberMap, ParameterExpression resolvedValue, Expression destinationMemberValue)
     {
         var typePair = memberMap.Types();
         var profile = _typeMap.Profile;

src/AutoMapper/TypeMap.cs

@@ -200,10 +200,7 @@ public void Seal(IGlobalConfiguration configuration)
         }
         _sealed = true;
         _details?.Seal(configuration, this);
-        if (!Projection)
-        {
-            MapExpression = CreateMapperLambda(configuration);
-        }
+        MapExpression = Projection ? EmptyLambda : CreateMapperLambda(configuration);
         SourceTypeDetails = null;
         DestinationTypeDetails = null;
     }

src/UnitTests/NullBehavior.cs

@@ -1,5 +1,34 @@
 namespace AutoMapper.UnitTests.NullBehavior;
-
+public class NullToExistingValue : AutoMapperSpecBase
+{
+    private record Person
+    {
+        public string Name { get; set; }
+        public Address TheAddress { get; set; } = new();
+    }
+    private record Address
+    {
+        public string Street { get; set; }
+        public int Number { get; set; }
+    }
+    private record PersonModel
+    {
+        public string Name { get; set; }
+        public AddressModel TheAddress { get; set; }
+    }
+    private record AddressModel
+    {
+        public string Street { get; set; }
+        public int Number { get; set; }
+    }
+    protected override MapperConfiguration CreateConfiguration() => new(c =>
+    {
+    	c.CreateMap<PersonModel, Person>();
+    	c.CreateMap<AddressModel, Address>();
+    });
+    [Fact]
+    public void Should_overwrite() => Mapper.Map(new PersonModel(), new Person()).TheAddress.ShouldBeNull();
+}
 public class NullCheckDefault : AutoMapperSpecBase
 {
     class Source