GCAllocator goodAllocSize?

[Steven Schveighoffer via Digitalmars-d]

import std.experimental.allocator.gc_allocator;

void main()
{
    //GCAllocator.instance.goodAllocSize(3000); // error
    writeln(theAllocator.goodAllocSize(3000)); // 3008, should be 4096
}

This is known info, why is it not available? Is there a reason this 
needs to be inaccurate? I can create a PR if desired.

-Steve