[Y_Y]

What are you going to put in that variable?

    void f();
    void v = f();
    void g(a){return a;}
    v = g(v);

Maybe my imagination is failing me, but I can't see how this can do much good without at least polymorphic functions.

[gpderetta]

   template<range R, regular X, invocable<range_value<R>, X> F>
     requires same_as<invoke_result_t<F, R, X>, X>
   auto fold(R&& range, F f, X accumulator) {
      for(auto x: range) 
         accumulator = f(x, accumulator);
      return accumulator;
   }

I can call that with a function returning a custom unit type:

   enum class void_t { Void };

   fold(my_range, [](auto&& elem, void_t) { return Void; }, Void);

But not with void:

   fold(my_range, [](auto&& elem, void) { return; }, void{});

which is very annoying and requires fold to special case 'void' via metaprogramming.

[remexre]

Templates already provide that useful polymorphism;

    template<typename T>
    T callWithState(auto f) {
        auto old = globalState;
        globalState = whatever();
        T out = f();
        globalState = old;
        return out;
    }

(forgive any syntax errors, my C++ is very rusty...)

[masklinn]

The void value. E.g.

    fn f() {}
    let mut v: () = f();
    fn g(a: ()) { a }
    V = g(v);

> Maybe my imagination is failing me, but I can't see how this can do much good without at least polymorphic functions.

Sure, it doesn’t do much useful to C save make void ever so slightly less wonky.