What is 'scope' in function parameter?

[Sobaya]

What means 'scope' in function parameter?

I made a test code.

```
import std.stdio;

int[] x;

void func(scope int[] a) {
     x = a;
}

void main() {
     func([0,1,2]);
     writeln(x);
}
```

This code was successfully compiled and printed '[0, 1, 2]'.

But according to https://dlang.org/spec/function.html, above code 
must cause a compile error.

Could you give me any advice?