Puzzled by this behavior

[Stefan Koch]

On Tuesday, 31 May 2022 at 17:41:18 UTC, Don Allen wrote:
> 
> ````
> import std.stdio;
>
> int main(string[] args)
> {
>     void foo() {
>         bar();
>     }
>     void bar() {
>         writeln("Hello world");
>     }
>     foo();
>     return 0;
> }
> ````

This is because you did it in a function body.
Within a function body each declaration opens a scope implicitly.
which gets closed at the end of the parent scope.
This makes sure you don't use variables before they got 
initialized.
```D
int f()
{
// {
   int x;
//   {
   int y = x;  // works
//     {
   int y2 = z;  // does not work because z not in scope
                 // also the value of z isn't defined yet
//       {
   int z;
   // } } } }
}
```
function declarations within a function body follow the same 
rules as variable declarations.
namely a declaration cannot forward reference another in the same 
function body.