Using emplace leading to memory corruption

[lomereiter]

You didn't tell GC that the allocated memory contains pointers to 
GC'd data.
Therefore, it thinks that the class members can be freed, which 
is not the case.

Adding lines
> import core.memory; GC.addRange(tmp, size);
resolves the problem. You must also call GC.removeRange(tmp) when 
you will free the allocated memory.

>       void[] mem = tmp[0..size];                           // 39
>       _foo = emplace!(Foo)(mem);                           // 40