Sorry, but I can't reproduce this locally on macOS 10.15.7 and Xcode 12.2. After changing platform target to 10.15 and signing with my developer account team, I'm able to build and run without a problem:

digraph {
  a -> b
  a -> c
  b -> c [constraint=false]
}
Program ended with exit code: 0

The error you're seeing suggests that Xcode is having problems with Foundation, the framework that provides the Process APIs. #10 updates CI to test Big Sur as well, to make sure this isn't an issue specific to the new OS release.

With platform target 10.15, It builds successfully.
It's an acceptable workaround for me right now (waiting for fixes on Big Sur side).

Thanks a lot for the time you spent on this.

@maxwacker FWIW, CI is passing on Big Sur, so it may be something specific to your project or difference between Swift Package Manager and conventional Xcode projects.

Closing this for now, unless you have any other updates. I'd gladly accept a PR with a failing test case for any problems you identify.