`this` template params for struct not expressing constness.

[adnan338]

Hi, as far as I understand, the `this` template parameter 
includes constness qualifiers as seen in 
https://ddili.org/ders/d.en/templates_more.html

To apply this I have this following struct:

module bst;

struct Tree(T) {
     T item;
     Tree!T* parent, left, right;

     this(T item) {
         this.item = item;
     }

     Self* searchTree(this Self)(auto in ref T item) const {
         if (&this is null)
             return null;
         if (this.item == item)
             return &this;
         return (this.item < item) ? this.right.searchTree(item) : 
this.right.searchTree(item);
     }
}

unittest {
     auto text1 = "Hello", text2 = "World";

     auto tree2 = Tree!string(text1);
     assert(tree2.searchTree(text2) is null);
     assert(tree2.searchTree(text1) !is null);

     auto tree1 = Tree!int(4);
     assert(tree1.searchTree(5) is null);
     assert(tree1.searchTree(4) !is null);

}


When I run the unittest the compiler complains:

cannot implicitly convert expression &this of type 
const(Tree!string)* to Tree!string*

Run link: https://run.dlang.io/is/b76DND

Why does this happen?