Currently a target B can include headers from a dependency C in its own public headers, but not expose this dependency in its public deps.

This is a bug because another target A that depends on B will see C's headers without having a public dependency, and it won't get the include directories and #defines that may be required to compile those headers.

There is a comment at the bottom HeaderChecker::CheckInclude in tools/gn/header_checker.cc with some ideas for detecting this.