Collection element type can not be set via `ICollectionConvention`

[cremor]

It seems like it is impossible to specify the type of a collection of elements in a convention.

I have written a convention which can be simplified to this:

public class MyElementConvention : ICollectionConvention {
   public void Apply(ICollectionInstance instance) {
      instance.Element.Type(typeof(MyUserType));
   }
}

It should be used in collection mappings like this:

HasMany(x => x.SomeElements)
   .Table("MyElements")
   .AsSet()
   .KeyColumn("ParentId")
   .Element("ElementValue");
   // Alternative without convention:
   //.Element("ElementValue", x => x.Type<MyUserType>());

The convention is executed correctly, I've confirmed this via a debugger. But the instance.Element.Type(mapperType) call in the Apply method never has any effect on the mapping.

I've debugged Fluent NHibernate a bit and it seems like the problem is here:

fluent-nhibernate/src/FluentNHibernate/Mapping/ToManyBase.cs

Lines 417 to 426 in eed20a2

	public T Element(string columnName)
	{
	elementPart = new ElementPart(typeof(T));
	elementPart.Type<TChild>();
	if (!string.IsNullOrEmpty(columnName))
	elementPart.Column(columnName);
	return (T)this;
	}

Line 420 already sets the "Type" attribute with Layer.UserSupplied, see:

fluent-nhibernate/src/FluentNHibernate/Mapping/ElementPart.cs

Lines 41 to 45 in eed20a2

	public ElementPart Type<TElement>()
	{
	attributes.Set("Type", Layer.UserSupplied, new TypeReference(typeof(TElement)));
	return this;
	}

But here the convention sets it with Layer.Conventions:

fluent-nhibernate/src/FluentNHibernate/Conventions/Instances/ElementInstance.cs

Lines 33 to 36 in eed20a2

	public new void Type(Type type)
	{
	mapping.Set(x => x.Type, Layer.Conventions, new TypeReference(type));
	}

So the attribute value from the convention isn't used.

I think the Element mapping method needs to use Layer.Defaults to allow the convention to work. So it should not call the public Type<TElement> method.