A Riddle: what is wrong with this code using std.array.Appender?

[Meta]

On Monday, 25 March 2019 at 17:32:13 UTC, Steven Schveighoffer 
wrote:
>> I can't see the bug in this minimal example:
>> 
>> import std.array: Appender;
>> 
>> class Class
>> {
>>      Appender!(int[]) app = null;
>> }
>> 
>> void main()
>> {
>>      auto c = new Class();
>>      import std.stdio;
>>      writeln(c.app.data); //Prints "[]"
>>      c.app ~= 10;
>>      writeln(c.app.data); //Prints "[10]"
>> }
>> 
>> What's the problem?
>
> I have a feeling it's an aliasing thing -- like every app 
> member in every class points at the same IMPL struct.
>
> -Steve

Ouch, you're right.

import std.array: Appender;

class Class
{
     Appender!(int[]) app = null;
}

void testAppender(Class c)
{
     import std.stdio;
     writeln(c.app.data);
     c.app ~= 10;
     writeln(c.app.data);
}

void main()
{
     auto c1 = new Class();
     auto c2 = new Class();
     testAppender(c1);  //Prints [] and [10]
     testAppender(c2); //Prints [10] and [10, 10]
}

I find that a bit strange, since you'd think that Appender would 
initialize its payload on the first append; and it seems like it 
does if you look at the code (in Appender.ensureAddable). I'm not 
sure how it shakes out that the two Appenders end up sharing the 
same memory.