Why can I not have circular exec_depend?

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I ran into a problem because of a circular exec_depend. Using catkin-tools, I get the following error:

[build] Error: The workspace packages have a circular dependency, and cannot be built. Please run catkin list --deps to determine the problematic package(s).

I completely understand why I cannot have circular build dependencies but, according to my understanding, an exec dependency means that the package needs the dependency only at run-time, so it should be possible to build it independently. So there should be no need to even look at the exec_depends at build time.

Is there something wrong in my understanding of exec_depend or is this a bug?