Added test that should trigger a memory leak.

tests/test_cpp.cc

@@ -939,6 +939,17 @@ void TestArena() {
     upb_arena_malloc(arena.ptr(), 1000000);
   }
   ASSERT(n == 0);
+
+  {
+    // Test fuse.
+    upb::Arena arena1;
+    upb::Arena arena2;
+
+    arena1.Fuse(arena2);
+
+    upb_arena_malloc(arena1.ptr(), 1);
+    upb_arena_malloc(arena2.ptr(), 1);
+  }
 }
 
 extern "C" {

upb/upb.hpp

@@ -60,6 +60,8 @@ class Arena {
     });
   }
 
+  void Fuse(Arena& other) { upb_arena_fuse(ptr(), other.ptr()); }
+
  private:
   std::unique_ptr<upb_arena, decltype(&upb_arena_free)> ptr_;
 };