Fix inner/outer ring accessor boost::geometry adaptor types/functions

These types/functions are used to let boost functions access our
polygon/ring types. They are supposed to return (const) references.

I found the bug while doing more complicated geometry processing,
working on some future code for osm2pgsql. It seems the buggy code does
not produce any problems in current code, but I can not guarantee it.
Theoretically it could lead to crashes. It might make some code slower,
though, because of unnecessary copying.

Author: joto

src/geom-boost-adaptor.hpp

@@ -54,29 +54,30 @@ struct ring_mutable_type<::geom::polygon_t>
 template <>
 struct interior_const_type<::geom::polygon_t>
 {
-    using type = std::vector<::geom::ring_t> const;
+    using type = std::vector<::geom::ring_t> const &;
 };
 template <>
 struct interior_mutable_type<::geom::polygon_t>
 {
-    using type = std::vector<::geom::ring_t>;
+    using type = std::vector<::geom::ring_t> &;
 };
 
 template <>
 struct exterior_ring<::geom::polygon_t>
 {
     // NOLINTNEXTLINE(google-runtime-references)
     static auto &get(::geom::polygon_t &p) { return p.outer(); }
-    static auto &get(::geom::polygon_t const &p) { return p.outer(); }
+    static auto const &get(::geom::polygon_t const &p) { return p.outer(); }
 };
 
 template <>
 struct interior_rings<::geom::polygon_t>
 {
     // NOLINTNEXTLINE(google-runtime-references)
-    static auto get(::geom::polygon_t &p) { return p.inners(); }
-    static auto get(::geom::polygon_t const &p) { return p.inners(); }
+    static auto &get(::geom::polygon_t &p) { return p.inners(); }
+    static auto const &get(::geom::polygon_t const &p) { return p.inners(); }
 };
+
 } // namespace boost::geometry::traits
 
 #endif // OSM2PGSQL_GEOM_BOOST_ADAPTOR_HPP