Currently this doesn't compile:

   while (auto parent = foo.parent)
   {
       foo = parent;
   }

Error: expression expected, not 'auto'

This is useful to replace this kind of loop (assume foo is a class object that has a 'parent' field):

   while (true)
   {
       if (auto parent = foo.parent)
       {
           // do something with parent
           foo = parent;  // switch to next parent
       }
       else
       {
           break;
       }
   }

with the simpler:
   
   while (auto parent = foo.parent)
   {
       // do something with parent
       foo = parent;  // switch to next parent
   }

Currently `if` statements already allow this syntax, so it seems natural to extend this to `while` loops.