Reported by Gary Willoughby, reduced by anonymous:

import core.memory;
void main()
{
     alias T = ubyte;

     enum size1 = 2_049; /* > 2_048 = 2^^11 */
     enum size2 = 1_048_577; /* > 1_048_576 = 2^^20 */

     T* _data;
     _data = cast(T*)GC.calloc(size1, GC.BlkAttr.NO_MOVE);
     _data = cast(T*)GC.realloc(_data, size2, GC.BlkAttr.NO_MOVE);

     T* _pointer = _data;
     foreach(i; 0 .. size2)
     {
         *_pointer = 0; /* segfault at i = 1_048_576 */
         _pointer++;
     }
}