Circle CI should generate a new build when a pull request is opened on github. The current implementation uses the branch's most recent commit result, instead of generating a brand new build.
Consider the following situation.
1) An engineer creates a branch within github, and pushes code up at their leisure. Circle CI builds the project on each commit.
2) When it comes time to make a pull request, the engineer opens up a PR on github, without creating a new git commit.
This workflow does not seem to trigger a new build of the branch on Circle CI, and instead Circle CI uses the build result of the most recent commit as the build status of the Pull Request. This seems to be an obvious workflow flaw given that I may have additional logic to build a PR artifact for testing purposes, vs the normal branch commit process. This has been discussed both here and here on your forums as well.
I have found 2 additional configurations under "advanced settings" that compliment this idea, but do not do what I intend.• Build forked pull requests• Only build pull requests
As a counterpoint, you can configure both Jenkins and Travis CI to do this, so this seems like a pretty straightforward use case of the platform. Obviously, if you if you push an additional commit to the branch that already has an existing PR, the additional variables or logic within Circle CI will be present.