Map<TSource, TDestination> should have class constraint for TDestination

[praschl]

In class TypeAdapterSetter<TSource, TDestination>, the following method

public TypeAdapterSetter<TSource, TDestination> Map<TDestinationMember, TSourceMember>(
    Expression<Func<TDestination, TDestinationMember>> member,
    Expression<Func<TSource, TSourceMember>> source,
    Expression<Func<TSource, bool>>? shouldMap = null)

and probably some more should have a "where TDestination: class" constraint.

We have an extension method that looks like this:

public static TypeAdapterSetter<Backend.Customer, TDestination> MapCustomer<TDestination>(this TypeAdapterSetter<Backend.Customer, TDestination> source)
    where TDestination : class, ICustomerData
{
    source
        .Map(dst => dst.Oid, src => src.Oid)
        // ...
        ;
}

if we remove the class constraint, the .Map() will throw the following exception:

System.ArgumentException: Allow only member access (eg. obj => obj.Child.Name) (Parameter 'lambda')
   at Mapster.Utils.ExpressionEx.GetMemberPath(LambdaExpression lambda, Boolean firstLevelOnly, Boolean noError)
   at Mapster.TypeAdapterSetter`2.Map[TDestinationMember,TSourceMember](Expression`1 member, Expression`1 source, Expression`1 shouldMap)

The Map() will work well as soon as we add the class constraint. (Mapping into structs also wouldn't make any sense, since that would only map into a copy.)

More methods might profit from the class constraint, but this is the one we had the issue with. If the method had a class constraint in the first place, we would have had to add it to our code to be able to even compile.