Surprising behaviour of std.experimental.allocator

[Saurabh Das]

This causes a segfault when run with rdmd -gx:

void main()
{
     import std.experimental.allocator : allocatorObject, 
expandArray;
     import 
std.experimental.allocator.building_blocks.allocator_list : 
AllocatorList;
     import std.experimental.allocator.building_blocks.region : 
Region;
     import 
std.experimental.allocator.building_blocks.fallback_allocator : 
FallbackAllocator;
     import std.experimental.allocator.mallocator : Mallocator;

     {
         alias Alloc1 = FallbackAllocator!(
             AllocatorList!(n => Region!Mallocator(1024*1024)),
             Mallocator);
         auto alloc1 = allocatorObject(Alloc1());
         for (int ttt=0; ttt<10; ++ttt)
         {
             import std.stdio : writeln;
             writeln(ttt);
             import core.memory : GC;

             auto p = alloc1.allocate(60*1024);
             assert(p.length == 60*1024);
             auto p2 = alloc1.allocate(3*1024*1024);
             assert(p2.length == 3*1024*1024);
             GC.collect();
             alloc1.expandArray(p, 120*1024); // Segfault here
         }
     }
}


(Tested on DMD 2.094.2 and on https://run.dlang.io/is/p0FsOQ)

If the "GC.collect()" line is commented out, it works somehow.

Please help me understand why this is happening. This is a very 
reduced example of an issue I am facing.

Thank You,
Saurabh