public partial class DerivedTypeMapper
    {
        [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
        public partial T MapGeneric<T>(object source)
        {
            return source switch
            {
                // --- Relevant Part ---
                global::MapperlyRepro.Fruit x when typeof(T).IsAssignableFrom(typeof(global::MapperlyRepro.FruitDto)) => (T)(object)MapDerivedTypes(x),
                _ => throw new global::System.ArgumentException($"Cannot map {source.GetType()} to {typeof(T)} as there is no known type mapping", nameof(source)),
            };
        }

        [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
        public partial global::MapperlyRepro.FruitDto MapDerivedTypes(global::MapperlyRepro.Fruit source)
        {
            return source switch
            {
                global::MapperlyRepro.Banana x => MapToBananaDto(x),
                _ => throw new global::System.ArgumentException($"Cannot map {source.GetType()} to MapperlyRepro.FruitDto as there is no known derived type mapping", nameof(source)),
            };
        }

        [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "4.3.1.0")]
        private global::MapperlyRepro.BananaDto MapToBananaDto(global::MapperlyRepro.Banana source)
        {
               ...
        }
    }

[Mapper]
public partial class DerivedTypeMapper
{
    public partial T MapGeneric<T>(object source);

    [MapDerivedType<Banana, BananaDto>]
    public partial FruitDto MapDerivedTypes(Fruit source);
}