Redirect tl_expected to system libexpected-dev when available

tl_expected/include/tl_expected/expected.hpp

@@ -13,6 +13,13 @@
 // <http://creativecommons.org/publicdomain/zero/1.0/>.
 ///
 
+// If the system header is available, redirect to it.
+// Otherwise, fall back to the vendored version with a deprecation warning.
+#if __has_include(<tl/expected.hpp>)
+#include <tl/expected.hpp>
+#else
+#warning "tl_expected/expected.hpp is deprecated. Please install libexpected-dev and use <tl/expected.hpp> instead."
+
 #ifndef TL_EXPECTED_HPP
 #define TL_EXPECTED_HPP
 
@@ -2480,4 +2487,6 @@ void swap(expected<T, E> &lhs,
 }
 } // namespace tl
 
-#endif
+#endif // TL_EXPECTED_HPP
+
+#endif // !defined(USE_VENDORED_TL_EXPECTED) && __has_include(<tl/expected.hpp>)

tl_expected/package.xml

@@ -14,6 +14,8 @@
 
   <buildtool_depend>ament_cmake</buildtool_depend>
 
+  <build_export_depend>libexpected-dev</build_export_depend>
+
   <export>
     <build_type>ament_cmake</build_type>
   </export>