Add non-leaf implementation

README.md

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+# Override non-leaf packages
+
+This is a repo with an example of two colcon workspaces: an underlay and an overlay.
+The underlay contains packages **a** and **b**, where **b** depends on **a**.
+The overlay contains packages **a** and **c** where **c** depends on **a**.
+
+Since **a** is non-leaf and overridden, it's changes must be ABI compatible or undefined behavior will occurr in any package that depends on both **b** and **c**.
+
+The packages here are pure CMake packages because I just want to see how colcon warns about them.
+
+## Workspace graphs
+
+This was generated with `colcon graph`.
+Arrows point from packages to their dependencies.
+
+
+### Overlay
+
+```dot
+digraph graphname {
+  "c";
+  "a";
+  "c" -> "a" [color="#0000ff:#ff0000"];
+}
+```
+
+
+### Underlay
+
+```dot
+digraph graphname {
+  "b";
+  "a";
+  "b" -> "a" [color="#0000ff:#ff0000"];
+}
+```

overlay/src/a/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.12)
+
+project(a VERSION 0.2.0 LANGUAGES C)
+
+include(CMakePackageConfigHelpers)
+
+set(LIB_INSTALL_DIR "lib" CACHE STRING "Where libs go")
+
+configure_package_config_file(a-config.cmake.in
+  ${CMAKE_CURRENT_BINARY_DIR}/a-config.cmake
+  INSTALL_DESTINATION ${LIB_INSTALL_DIR}/a/cmake)
+
+write_basic_package_version_file(
+  ${CMAKE_CURRENT_BINARY_DIR}/a-config-version.cmake
+  VERSION ${a_VERSION}
+  COMPATIBILITY SameMajorVersion)
+
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/a-config.cmake
+              ${CMAKE_CURRENT_BINARY_DIR}/a-config-version.cmake
+        DESTINATION ${LIB_INSTALL_DIR}/a/cmake)

overlay/src/a/a-config.cmake.in

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+@PACKAGE_INIT@
+
+check_required_components(a)

overlay/src/c/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.12)
+
+project(c VERSION 0.1.0 LANGUAGES C)
+
+find_package(a REQUIRED)
+
+include(CMakePackageConfigHelpers)
+
+set(LIB_INSTALL_DIR "lib" CACHE STRING "Where libs go")
+
+configure_package_config_file(c-config.cmake.in
+  ${CMAKE_CURRENT_BINARY_DIR}/c-config.cmake
+  INSTALL_DESTINATION ${LIB_INSTALL_DIR}/c/cmake)
+
+write_basic_package_version_file(
+  ${CMAKE_CURRENT_BINARY_DIR}/c-config-version.cmake
+  VERSION ${b_VERSION}
+  COMPATIBILITY SameMajorVersion)
+
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/c-config.cmake
+              ${CMAKE_CURRENT_BINARY_DIR}/c-config-version.cmake
+        DESTINATION ${LIB_INSTALL_DIR}/c/cmake)

overlay/src/c/c-config.cmake.in

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+@PACKAGE_INIT@
+
+check_required_components(b)

underlay/src/a/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.12)
+
+project(a VERSION 0.1.0 LANGUAGES C)
+
+include(CMakePackageConfigHelpers)
+
+set(LIB_INSTALL_DIR "lib" CACHE STRING "Where libs go")
+
+configure_package_config_file(a-config.cmake.in
+  ${CMAKE_CURRENT_BINARY_DIR}/a-config.cmake
+  INSTALL_DESTINATION ${LIB_INSTALL_DIR}/a/cmake)
+
+write_basic_package_version_file(
+  ${CMAKE_CURRENT_BINARY_DIR}/a-config-version.cmake
+  VERSION ${a_VERSION}
+  COMPATIBILITY SameMajorVersion)
+
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/a-config.cmake
+              ${CMAKE_CURRENT_BINARY_DIR}/a-config-version.cmake
+        DESTINATION ${LIB_INSTALL_DIR}/a/cmake)

underlay/src/a/a-config.cmake.in

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+@PACKAGE_INIT@
+
+check_required_components(a)

underlay/src/b/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.12)
+
+project(b VERSION 0.1.0 LANGUAGES C)
+
+find_package(a REQUIRED)
+
+include(CMakePackageConfigHelpers)
+
+set(LIB_INSTALL_DIR "lib" CACHE STRING "Where libs go")
+
+configure_package_config_file(b-config.cmake.in
+  ${CMAKE_CURRENT_BINARY_DIR}/b-config.cmake
+  INSTALL_DESTINATION ${LIB_INSTALL_DIR}/b/cmake)
+
+write_basic_package_version_file(
+  ${CMAKE_CURRENT_BINARY_DIR}/b-config-version.cmake
+  VERSION ${b_VERSION}
+  COMPATIBILITY SameMajorVersion)
+
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/b-config.cmake
+              ${CMAKE_CURRENT_BINARY_DIR}/b-config-version.cmake
+        DESTINATION ${LIB_INSTALL_DIR}/b/cmake)

underlay/src/b/b-config.cmake.in

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+@PACKAGE_INIT@
+
+check_required_components(b)