Please describe the problem:
The end_ptr parameter of the string.to_double should be weak.
Because strtod doesn't create any new string.

Steps to reproduce:
This code should work.

        protected override void from_string(string str){
            weak string s = str;
            weak string end_ptr = null;
            int dimensions = 0;
            while(true) {
                s.to_double(out end_ptr);
                if((void*) s == (void*) end_ptr) break;
                s = end_ptr;
                dimensions ++ ;
            }
            this.dimensions = dimensions;
            s = str;
            for(int i = 0; i< dimensions; i++){
                positions[i] = s.to_double(out s);
                dimensions ++ ;
            }
        }

But ends up with 
libgdakit/temp.vala:70.17-70.27: error: Invalid assignment from owned expression to unowned variable
                                s.to_double(out end_ptr);
                                            ^^^^^^^^^^^
libgdakit/temp.vala:78.32-78.36: error: Invalid assignment from owned expression to unowned variable
                                positions[i] = s.to_double(out s);
                                                           ^^^^^
Compilation failed: 2 error(s), 0 warning(s)


Actual results:


Expected results:


Does this happen every time?
Yes

Other information:
A workaround is to declare the strings as 'strong'. But it ends up with a messy ccode that duplicate the strings for many times.