This is mostly to prevent ugly hacks like Flag [1]. http://wiki.dlang.org/DIP88[1] https://dlang.org/phobos/std_typecons.html#.Flag
Why not simply allow programmer to declare auto variable at function call side like in `foreach`?
E.g.
```d
void foo(int width, int height){...} //function definition
foo(width = 2, height = 3); // function call
```
