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

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

test/set_test.hpp

@@ -580,11 +580,20 @@ int set_test ()
 
    //Compare count with std containers
    for(int i = 0; i < MaxElem; ++i){
-      IntType count_me(i);
-      if(boostset.count(count_me) != stdset.count(i)){
+      IntType k(i);
+      if(boostset.count(k) != stdset.count(i)){
          return -1;
       }
-      if(boostmultiset.count(count_me) != stdmultiset.count(i)){
+
+      if(boostset.contains(k) != (stdset.find(i) != stdset.end())){
+         return -1;
+      }
+
+      if(boostmultiset.count(k) != stdmultiset.count(i)){
+         return -1;
+      }
+
+      if(boostmultiset.contains(k) != (stdmultiset.find(i) != stdmultiset.end())){
          return -1;
       }
    }