import std.experimental.allocator.building_blocks.region : Region;
import std.experimental.allocator.gc_allocator : GCAllocator;
import std.typecons : Ternary;
auto a = fallbackAllocator(Region!GCAllocator(1024), GCAllocator.instance);
auto b1 = a.allocate(1020);
assert(b1.length == 1020);
assert(a.primary.owns(b1) == Ternary.yes);
auto b2 = a.allocate(10);
assert(b2.length == 10);
assert(a.primary.owns(b2) == Ternary.no);