toString contains null for struct with function/method

[Paul Backus]

On Sunday, 8 April 2018 at 15:04:49 UTC, number wrote:
> the write() shows a 'null' if the struct has a function/method. 
> why is that?
> ```
> import std.stdio;
>
> void main()
> {
> 	struct S
> 	{
> 		int i;
> 	}
> 	S s;
> 	writeln(s);                // S(0)
> 	writeln(typeid(s).sizeof); // 8
> 	
> 	struct S2
> 	{
> 		int i;
> 		this(this){}
> 	}
> 	S2 s2;
> 	import std.conv: to;
> 	writeln(s2);                // S2(0, null)
> 	writeln(typeid(s2).sizeof); // 8
> }
> ```

S2 is a nested struct [1], which means it has a hidden pointer 
field that's used to access its enclosing scope. If you change 
the definition to `static struct S2`, you'll get the same output 
for both structs.

[1] https://dlang.org/spec/struct.html#nested