Catch TypeError of non-duplicable objects

In Ruby versions prior to 2.4.x NilClass, Fixnum, Float and Symbol among others
where not cloneable/duplicable and threw a TypeError. In newer versions they
simply return self.

Since rescuing the TypeError is approximately 40x slower than using a predicate
to check whether an object is cloneable or not, one could consider to implement
a `duplicable?` method on Object and conditionally overwrite that on descendants.
See https://stackoverflow.com/questions/49321553/why-is-duplicable-defined-the-way-it-is

However, assuming that this code is not part of a hot code path, that seems
like an unnecessary performance improvement.

Author: kaikuchn

lib/concurrent/synchronization/abstract_struct.rb

@@ -117,7 +117,13 @@ def ns_merge(other, &block)
 
       # @!visibility private
       def ns_initialize_copy
-        @values = @values.map(&:clone)
+        @values = @values.map do |val|
+          begin
+            val.clone
+          rescue TypeError
+            val
+          end
+        end
       end
 
       # @!visibility private