Bug on dmd or gdc ?

[Domingo Alvarez Duarte via Digitalmars-d-learn]

Hello !

Based on a question about manually allocated structures with 
payload I found that a solution proposed seems to work when 
compiled with dmd but segfaults when compiled with gdc.

So my question is: Is this a bug on dmd, gdc or a bad idiom ?

---- code to see the problem
import std.stdio;
import core.stdc.stdlib;

void main()
{
	struct S
	{
		int size;
		char[0] _b;
		@property char[] buf() { return (_b.ptr)[0..size];}
	}
	
	int size = 12;
	S *s = cast(S*)malloc(S.sizeof + size);
	if(s !is null)
	{
		scope(exit) free(s);
		s.size = size;
		s.buf[0] = 'a';
		writeln(s.buf[0]);
	}
}