Allow iterator copy for lambda in transformed and filtered

Lambda has no default constructor nor a copy or move assignment.

range\test\adaptors.cpp is checking that produced ranges are copiable,
but it is not doing so the for lambda, or lambda like objects.

default_constructible_unary_fn uses an optional for default
construction.
I extended it to use its emplace facility to copy assign non copy
assignable types.
(see boost::optional 'Type Requierment':  'If T is not MoveAssignable,
it is still possible to reset the value of optional<T> using function
emplace():')

include/boost/range/detail/default_constructible_unary_fn.hpp

@@ -32,6 +32,23 @@ public:
         : m_impl(source)
     {
     }
+    default_constructible_unary_fn_wrapper(const default_constructible_unary_fn_wrapper& source)
+        : m_impl(source.m_impl)
+    {
+    }
+    default_constructible_unary_fn_wrapper& operator=(const default_constructible_unary_fn_wrapper& source)
+    {
+        if (source.m_impl)
+        {
+            // Lambda are not copy/move assignable.
+            m_impl.emplace(*source.m_impl);
+        }
+        else
+        {
+            m_impl.reset();
+        }
+        return *this;
+    }
     template<typename Arg>
     R operator()(const Arg& arg) const
     {

test/adaptor_test/filtered.cpp

@@ -51,6 +51,23 @@ namespace boost
             bool operator()( IntegerT x ) const { return x % 2 != 0; }
         };
 
+        struct lambda_init
+        {
+        };
+
+        struct lambda
+        {
+            lambda(const lambda_init& init) {}
+            lambda(const lambda& rhs) {}
+
+            template< class T1 >
+            bool operator()(T1) const { return false; }
+
+        private:
+            lambda() {}
+            lambda& operator=(const lambda& rhs) { return *this; }
+        };
+
         template< class Container, class Pred >
         void filtered_test_impl( Container& c, Pred pred )
         {
@@ -86,32 +103,53 @@ namespace boost
                                            test_result2.end() );
         }
 
+        template< class Rng >
+        void check_copy_assign(Rng r)
+        {
+            Rng r2 = r;
+            r2 = r;
+        }
+
         template< class Container, class Pred >
+        void filtered_range_copy_assign(Container& c, Pred pred)
+        {
+            using namespace boost::adaptors;
+            check_copy_assign(c | filtered(pred));
+            check_copy_assign(adaptors::filter(c, pred));
+        }
+
+        template< class Container, class Pred, class PredInit >
         void filtered_test_impl()
         {
             using namespace boost::assign;
 
             Container c;
+            PredInit init;
+            Pred pred(init);
 
             // test empty container
-            filtered_test_impl(c, Pred());
+            filtered_test_impl(c, pred);
 
             // test one element
             c += 1;
-            filtered_test_impl(c, Pred());
+            filtered_test_impl(c, pred);
 
             // test many elements
             c += 1,2,2,2,3,4,4,4,4,5,6,7,8,9,9;
-            filtered_test_impl(c, Pred());
+            filtered_test_impl(c, pred);
+
+            // test the range and iterator are copy assignable
+            filtered_range_copy_assign(c, pred);
         }
 
         template< class Container >
         void filtered_test_all_predicates()
         {
-            filtered_test_impl< Container, always_false_pred >();
-            filtered_test_impl< Container, always_true_pred >();
-            filtered_test_impl< Container, is_odd >();
-            filtered_test_impl< Container, is_even >();
+            filtered_test_impl< Container, always_false_pred, always_false_pred >();
+            filtered_test_impl< Container, always_true_pred, always_true_pred >();
+            filtered_test_impl< Container, is_odd, is_odd >();
+            filtered_test_impl< Container, is_even, is_even >();
+            filtered_test_impl< Container, lambda, lambda_init >();
         }
 
         void ticket_10988_single_pass()

test/adaptor_test/transformed.cpp

@@ -38,6 +38,22 @@ namespace boost
             int operator()(int x) const { return x / 2; }
         };
 
+        struct lambda_init
+        {
+        };
+
+        struct lambda
+        {
+            lambda(const lambda_init& init) {}
+            lambda(const lambda& rhs) {}
+
+            int operator()(int x) const { return x + 1; }
+
+        private:
+            lambda() {}
+            lambda& operator=(const lambda& rhs) { return *this; }
+        };
+
         template< class Container, class TransformFn >
         void transformed_test_impl_core( Container& c, TransformFn fn )
         {
@@ -59,13 +75,29 @@ namespace boost
                                            test_result2.begin(), test_result2.end() );
         }
 
+        template< class Rng >
+        void check_copy_assign(Rng r)
+        {
+            Rng r2 = r;
+            r2 = r;
+        }
+
         template< class Container, class TransformFn >
+        void transformed_range_copy_assign(Container& c, TransformFn fn)
+        {
+            using namespace boost::adaptors;
+            check_copy_assign(c | transformed(fn));
+            check_copy_assign(adaptors::transform(c, fn));
+        }
+
+        template< class Container, class TransformFn, class TransformFnInit >
         void transformed_test_fn_impl()
         {
             using namespace boost::assign;
 
             Container c;
-            TransformFn fn;
+            TransformFnInit init;
+            TransformFn fn( init );
 
             // Test empty
             transformed_test_impl_core(c, fn);
@@ -77,13 +109,17 @@ namespace boost
             // Test many elements
             c += 1,1,1,2,2,2,2,2,3,4,5,6,7,8,9;
             transformed_test_impl_core(c, fn);
+
+            // test the range and iterator are copy assignable
+            transformed_range_copy_assign(c, fn);
         }
 
         template< class Container >
         void transformed_test_impl()
         {
-            transformed_test_fn_impl< Container, double_x >();
-            transformed_test_fn_impl< Container, halve_x >();
+            transformed_test_fn_impl< Container, double_x, double_x >();
+            transformed_test_fn_impl< Container, halve_x, halve_x >();
+            transformed_test_fn_impl< Container, lambda, lambda_init >();
         }
 
         void transformed_test()