This compiles and runs just fine, printing what you'd except:

@safe void main()
{
    import std.stdio;

    int* p;
    isNull(*p).writeln;
    p = new int(5);
    isNull(*p).writeln;
}

@safe bool isNull(ref int x){return &x is null;}

The question is, is this intentional? I'm writing "DIP1000: Memory Safety in a Modern System Programming Language Pt. 2" and currently I've written that this is allowed, even when it's not in C++.

But there are still two possibilities: either this is okay by design, or works by accident but really disallowed. Which is the case?

This compiles and runs just fine, printing what you'd except:

@safe void main()
{
    import std.stdio;

    int* p;
    isNull(*p).writeln;
    p = new int(5);
    isNull(*p).writeln;
}

@safe bool isNull(ref int x){return &x is null;}

The question is, is this intentional? I'm writing "DIP1000: Memory Safety in a Modern System Programming Language Pt. 2" and currently I've written that this is allowed, even when it's not in C++.

It doens't compile when I run it on run.dlang.io, erroring out with:

onlineapp.d(11): Error: cannot take address of parameter `x` in `@safe` function `isNull`