Can I use a publication's accompanying source code in a commercial software? Said source code has no explicit license.
I know I could reimplement the method described in the paper, but it would be more convenient to just use the code directly.
What is the convention in these situations? Since methods aren't copyrightable, but source code is usually copyrighted by default.
P.S.: There's no patent on the method.
Said source code has no explicit license.


Let's see what licence means:


  a grant by the holder of a copyright or patent to another of any of the rights embodied in the copyright or patent short of an assignment of all rights


If there is no licence, then you are not allowed by the copyright holder to use the code. It's as simple as that. There is nothing that requires anyone to grant a licence to anyone else to use their code, and if they don't then no-one besides the copyright holder can legally use the code. It doesn't matter if the code is public.

Maybe this analogy will help: imagine I paint something and decide to hang the painting in a public space. Can anyone come along and start copying my painting because "there is no explicit licence"? Absolutely not! I hold the copyright to the painting, and you cannot copy, reuse... my painting without my permission. Well, as you say the code is copyrighted too, and there is no "default licence" that allows anyone to come along and copy the code unless stated otherwise; quite the opposite, actually.

Of course, you can always ask the copyright holder for permission, but check who the copyright holder actually is (it may not be the publication's authors anymore).