Be able to use custom conversion method using that maps to existing object in sub mapping

[PaulVrugt]

I've been experimenting with mapperly, and we seriously consider dropping automapper an migrating (in steps) to mapperly. All kinds of stuff that I though wouldn't work in my test project actually already works out of the box. My compliments for this. This is a very cool library.

One thing that doesn't seem the work is the following regarding User implemented mapping methods:

Say I have a class:

public class SourceClass
{
    public SubSourceClass SubEntity {get; set; }
}

public class TargetClass
{
    public required TargetSubClass SubEntity {get; set; } = new();
}

and in my mapper I have a converter that maps to an existing TargetClass, and a custom converter for SubSourceClass:

//mapping method to existing TargetClass
public partial void ToTargetClass(SourceClass source, TargetClass target);
//custom mapping function to existing TargetSubClass
public static void ToTargetSubClass(SubSourceClass source, TargetSubClass target) {}

It would be really cool to be able to have mapperly recognize that when generating the mapping, it can use the ToTargetSubClass method, since I'm mapping to an existing TargetClass and the SubEntity property already has a value (either by checking the value, or in this case even because the SubEntity property is a non-nullable required property

[latonz]

With 3.1.0 this should work if the TargetClass.SubEntity property is not settable. It does not work for required settable properties, as Mapperly thinks it can just map the value to a new instance and assign that.