* Implemented C++20 contains() for associative containers as specified in P0458R2.

* Fixed serious bug in heterogeneous lookup functions (is_transparent was broken).

include/boost/container/map.hpp

@@ -1119,6 +1119,22 @@ class map
 
    #if defined(BOOST_CONTAINER_DOXYGEN_INVOKED)
 
+   //! <b>Returns</b>: Returns true if there is an element with key
+   //!   equivalent to key in the container, otherwise false.
+   //!
+   //! <b>Complexity</b>: log(size()).
+   bool contains(const key_type& x) const;
+
+   //! <b>Requires</b>: This overload is available only if
+   //! key_compare::is_transparent exists.
+   //!
+   //! <b>Returns</b>: Returns true if there is an element with key
+   //!   equivalent to key in the container, otherwise false.
+   //!
+   //! <b>Complexity</b>: log(size()).
+   template<typename K>
+   bool contains(const K& x) const;
+
    //! <b>Returns</b>: An iterator pointing to the first element with key not less
    //!   than k, or a.end() if such an element is not found.
    //!
@@ -2017,6 +2033,22 @@ class multimap
    template<typename K>
    size_type count(const K& x) const;
 
+   //! <b>Returns</b>: Returns true if there is an element with key
+   //!   equivalent to key in the container, otherwise false.
+   //!
+   //! <b>Complexity</b>: log(size()).
+   bool contains(const key_type& x) const;
+
+   //! <b>Requires</b>: This overload is available only if
+   //! key_compare::is_transparent exists.
+   //!
+   //! <b>Returns</b>: Returns true if there is an element with key
+   //!   equivalent to key in the container, otherwise false.
+   //!
+   //! <b>Complexity</b>: log(size()).
+   template<typename K>
+   bool contains(const K& x) const;
+
    //! <b>Returns</b>: An iterator pointing to the first element with key not less
    //!   than k, or a.end() if such an element is not found.
    //!