This is the exact thing I'm not sure about. See https://news.ycombinator.com/item?id=48470397 where I posit a simpler question: if a `test_sum()` function is copyrighted, does writing a `sum(a, b)` function infringe on the copyright of the software product that `test_sum()` is a part of. I'd say no. There's another part of the GPL that applies here:

> A compilation of a covered work with other separate and independent works, which are not by their nature extensions of the covered work, and which are not combined with it such as to form a larger program, in or on a volume of a storage or distribution medium, is called an “aggregate” if the compilation and its resulting copyright are not used to limit the access or legal rights of the compilation's users beyond what the individual works permit. Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate.

So assuming that sum(a, b) is non-infringing and not combined to form a larger program (i.e. the tests aren't compiled into the grit code), then the GPL explicitly doesn't apply to this use

test_sum is assumedly relatively trivial. So as a lay person I’d expect some sort of obviousness test to apply. Like so much of the stuff in the Google/Oracle lawsuit.

But if you take all the individual tests used to test git as a whole, that seems far more unique. Seems like at that point you’re really having to duplicate the actual git internals, and that seems like it should be covered.