----
import std.stdio;
@safe:
void main()
{
	writeln("Edit source/app.d to start your project.");
}

----
source/app.d(5): Error: safe function 'D main' cannot call system
function 'std.stdio.writeln!(string).writeln'

'cause `writeln`() is not @safe, as you can read in the following
compiler message:

> source/app.d(5): Error: safe function 'D main' cannot call system
> function 'std.stdio.writeln!(string).writeln'

On Saturday, November 29, 2014 07:24:55 Freddy via Digitalmars-d-learn wrote:
> ----
> import std.stdio;
> @safe:
> void main()
> {
>   writeln("Edit source/app.d to start your project.");
> }
>
> ----
> source/app.d(5): Error: safe function 'D main' cannot call system
> function 'std.stdio.writeln!(string).writeln'

Well, as the error message says, writeln isn't @safe. It's @system. Now, in this case, writeln really should be inferred as @safe, but it looks like whatever work needs to be done to make it so that writeln is treated as @safe when it can be hasn't been done yet (whether it can be or not depends on what's passed to it, so implementing that properly could be rather complicated, but mainly, the issue is that someone needs to take the time to do it).

- Jonathan M Davis

Freddy:

> ----
> import std.stdio;
> @safe:
> void main()
> {
> 	writeln("Edit source/app.d to start your project.");
> }
>
> ----
> source/app.d(5): Error: safe function 'D main' cannot call system
> function 'std.stdio.writeln!(string).writeln'

With the latest dmd compiler this compiles fine.

Bye,
bearophile