For debugging things, like in unittest, and like in the auto test suite, I like the test code to be as self-contained as possible.

In this case, with a trivial copy(), I was able to identify the source of the array overflow immediately. I was looking at (possibly) an hour of work otherwise - it certainly wasn't immediately obvious where the problem was, as the overflow message pointed into one of the other Phobos modules (not std.algorithm).