package_d -> package_b

package_d -> package_c

package_c -> package_a

To reproduce a dependency issue, build an underlay workspace with a merge install space

    cd underlay
    colcon build --merge-install

Source the underlay and build the overlay

    source install/setup.bash
    cd ../overlay
    colcon build

Expect a compiler error due to an undeclared function package_a_foo_new.

package_a exists in both the underlay and overlay, with a modified version in the overlay. package_c depends on the modified version of package_a; a sibling in the overlay. package_d depends on package_c and package_b (a package in the underlay).

package_d happens to order the include directories such that package_b appears before package_c. Since the underlay is a merge install, the include directoy for package_b is the same as for package_a. Unfortunately, this means that package_b will include headers from package_a in the underlay instead of the overlay.