struct visibility attributes

[eugene via Digitalmars-d]

Hello, everyone,
i looked at 
https://dlang.org/spec/attribute.html#visibility_attributes but 
it says nothing about visibility attributes in structs.
The question is: why is the code compiled by ldc and working:
import std.stdio:writeln;
void main()
{
    test_struct testStruct = test_struct();
    testStruct.add_to_x; // add_to_x is private
    testStruct.add_to_x;
    writeln("x=", testStruct.x); // x is private
}
struct test_struct
{
    private:
        int x;
        void add_to_x()
        {
            x++;
        }
        
    public:
        int get_x()
        {
            return x;
        }
}