Index: include/bits/hashtable_policy.h
===================================================================
--- include/bits/hashtable_policy.h	(revision 213780)
+++ include/bits/hashtable_policy.h	(working copy)
@@ -460,7 +460,7 @@
   /// smallest prime that keeps the load factor small enough.
   struct _Prime_rehash_policy
   {
-    _Prime_rehash_policy(float __z = 1.0)
+    _Prime_rehash_policy(float __z = 1.0) noexcept
     : _M_max_load_factor(__z), _M_next_resize(0) { }
 
     float
@@ -1071,7 +1071,8 @@
       typedef void* 					__hash_code;
       typedef _Hash_node<_Value, false>			__node_type;
 
-      // We need the default constructor for the local iterators.
+      // We need the default constructor for the local iterators and _Hashtable
+      // default constructor.
       _Hash_code_base() = default;
 
       _Hash_code_base(const _ExtractKey& __ex, const _H1&, const _H2&,
@@ -1161,7 +1162,8 @@
       typedef std::size_t 				__hash_code;
       typedef _Hash_node<_Value, false>			__node_type;
 
-      // We need the default constructor for the local iterators.
+      // We need the default constructor for the local iterators and _Hashtable
+      // default constructor.
       _Hash_code_base() = default;
 
       _Hash_code_base(const _ExtractKey& __ex,
@@ -1250,6 +1252,8 @@
       typedef std::size_t 				__hash_code;
       typedef _Hash_node<_Value, true>			__node_type;
 
+      // We need the default constructor for _Hashtable default constructor.
+      _Hash_code_base() = default;
       _Hash_code_base(const _ExtractKey& __ex,
 		      const _H1& __h1, const _H2& __h2,
 		      const _Default_ranged_hash&)
@@ -1694,6 +1698,7 @@
 					__hash_code, __hash_cached::value>;
 
   protected:
+    _Hashtable_base() = default;
     _Hashtable_base(const _ExtractKey& __ex, const _H1& __h1, const _H2& __h2,
 		    const _Hash& __hash, const _Equal& __eq)
     : __hash_code_base(__ex, __h1, __h2, __hash), _EqualEBO(__eq)
@@ -1906,6 +1911,7 @@
 	__alloc_rebind<__node_alloc_type, __bucket_type>;
       using __bucket_alloc_traits = std::allocator_traits<__bucket_alloc_type>;
 
+      _Hashtable_alloc() = default;
       _Hashtable_alloc(const _Hashtable_alloc&) = default;
       _Hashtable_alloc(_Hashtable_alloc&&) = default;
 
Index: include/bits/hashtable.h
===================================================================
--- include/bits/hashtable.h	(revision 213780)
+++ include/bits/hashtable.h	(working copy)
@@ -310,10 +310,10 @@
 				   const_local_iterator;
 
     private:
-      __bucket_type*		_M_buckets;
-      size_type			_M_bucket_count;
+      __bucket_type*		_M_buckets		= &_M_single_bucket;
+      size_type			_M_bucket_count		= 1;
       __node_base		_M_before_begin;
-      size_type			_M_element_count;
+      size_type			_M_element_count	= 0;
       _RehashPolicy		_M_rehash_policy;
 
       // A single bucket used when only need for 1 bucket. Especially
@@ -322,7 +322,7 @@
       // qualified.
       // Note that we can't leave hashtable with 0 bucket without adding
       // numerous checks in the code to avoid 0 modulus.
-      __bucket_type		_M_single_bucket;
+      __bucket_type		_M_single_bucket	= nullptr;
 
       bool
       _M_uses_single_bucket(__bucket_type* __bkts) const
@@ -382,8 +382,16 @@
       void
       _M_reset() noexcept;
 
+      _Hashtable(const _H1& __h1, const _H2& __h2, const _Hash& __h,
+		 const _Equal& __eq, const _ExtractKey& __exk,
+		 const allocator_type& __a)
+	: __hashtable_base(__exk, __h1, __h2, __h, __eq),
+	  __hashtable_alloc(__node_alloc_type(__a))
+      { }
+
     public:
       // Constructor, destructor, assignment, swap
+      _Hashtable() = default;
       _Hashtable(size_type __bucket_hint,
 		 const _H1&, const _H2&, const _Hash&,
 		 const _Equal&, const _ExtractKey&,
@@ -407,12 +415,11 @@
       // Use delegating constructors.
       explicit
       _Hashtable(const allocator_type& __a)
-      : _Hashtable(10, _H1(), _H2(), _Hash(), key_equal(),
-		   __key_extract(), __a)
+	: __hashtable_alloc(__node_alloc_type(__a))
       { }
 
       explicit
-      _Hashtable(size_type __n = 10,
+      _Hashtable(size_type __n,
 		 const _H1& __hf = _H1(),
 		 const key_equal& __eql = key_equal(),
 		 const allocator_type& __a = allocator_type())
@@ -791,15 +798,14 @@
 	       const _H1& __h1, const _H2& __h2, const _Hash& __h,
 	       const _Equal& __eq, const _ExtractKey& __exk,
 	       const allocator_type& __a)
-    : __hashtable_base(__exk, __h1, __h2, __h, __eq),
-      __map_base(),
-      __rehash_base(),
-      __hashtable_alloc(__node_alloc_type(__a)),
-      _M_element_count(0),
-      _M_rehash_policy()
+      : _Hashtable(__h1, __h2, __h, __eq, __exk, __a)
     {
-      _M_bucket_count = _M_rehash_policy._M_next_bkt(__bucket_hint);
-      _M_buckets = _M_allocate_buckets(_M_bucket_count);
+      auto __bkt = _M_rehash_policy._M_next_bkt(__bucket_hint);
+      if (__bkt > _M_bucket_count)
+	{
+	  _M_buckets = _M_allocate_buckets(__bkt);
+	  _M_bucket_count = __bkt;
+	}
     }
 
   template<typename _Key, typename _Value,
@@ -814,20 +820,20 @@
 		 const _H1& __h1, const _H2& __h2, const _Hash& __h,
 		 const _Equal& __eq, const _ExtractKey& __exk,
 		 const allocator_type& __a)
-      : __hashtable_base(__exk, __h1, __h2, __h, __eq),
-	__map_base(),
-	__rehash_base(),
-	__hashtable_alloc(__node_alloc_type(__a)),
-	_M_element_count(0),
-	_M_rehash_policy()
+	: _Hashtable(__h1, __h2, __h, __eq, __exk, __a)
       {
 	auto __nb_elems = __detail::__distance_fw(__f, __l);
-	_M_bucket_count =
+	auto __bkt_count =
 	  _M_rehash_policy._M_next_bkt(
 	    std::max(_M_rehash_policy._M_bkt_for_elements(__nb_elems),
 		     __bucket_hint));
 
-	_M_buckets = _M_allocate_buckets(_M_bucket_count);
+	if (__bkt_count > _M_bucket_count)
+	  {
+	    _M_buckets = _M_allocate_buckets(__bkt_count);
+	    _M_bucket_count = __bkt_count;
+	  }
+
 	__try
 	  {
 	    for (; __f != __l; ++__f)
@@ -1101,7 +1107,7 @@
       __rehash_base(__ht),
       __hashtable_alloc(
 	__node_alloc_traits::_S_select_on_copy(__ht._M_node_allocator())),
-      _M_buckets(),
+      _M_buckets(nullptr),
       _M_bucket_count(__ht._M_bucket_count),
       _M_element_count(__ht._M_element_count),
       _M_rehash_policy(__ht._M_rehash_policy)
@@ -1175,7 +1181,7 @@
       __map_base(__ht),
       __rehash_base(__ht),
       __hashtable_alloc(__node_alloc_type(__a)),
-      _M_buckets(),
+      _M_buckets(nullptr),
       _M_bucket_count(__ht._M_bucket_count),
       _M_element_count(__ht._M_element_count),
       _M_rehash_policy(__ht._M_rehash_policy)
@@ -1218,8 +1224,7 @@
     ~_Hashtable() noexcept
     {
       clear();
-      if (_M_buckets)
-	_M_deallocate_buckets();
+      _M_deallocate_buckets();
     }
 
   template<typename _Key, typename _Value,
Index: include/bits/unordered_map.h
===================================================================
--- include/bits/unordered_map.h	(revision 213780)
+++ include/bits/unordered_map.h	(working copy)
@@ -128,15 +128,18 @@
 
       //construct/destroy/copy
 
+      /// Default constructor.
+      unordered_map() = default;
+
       /**
        *  @brief  Default constructor creates no elements.
-       *  @param __n  Initial number of buckets.
+       *  @param __n  Minimal initial number of buckets.
        *  @param __hf  A hash functor.
        *  @param __eql  A key equality functor.
        *  @param __a  An allocator object.
        */
       explicit
-      unordered_map(size_type __n = 10,
+      unordered_map(size_type __n,
 		    const hasher& __hf = hasher(),
 		    const key_equal& __eql = key_equal(),
 		    const allocator_type& __a = allocator_type())
@@ -840,15 +843,18 @@
 
       //construct/destroy/copy
 
+      /// Default constructor.
+      unordered_multimap() = default;
+
       /**
        *  @brief  Default constructor creates no elements.
-       *  @param __n  Initial number of buckets.
+       *  @param __n  Mnimal initial number of buckets.
        *  @param __hf  A hash functor.
        *  @param __eql  A key equality functor.
        *  @param __a  An allocator object.
        */
       explicit
-      unordered_multimap(size_type __n = 10,
+      unordered_multimap(size_type __n,
 			 const hasher& __hf = hasher(),
 			 const key_equal& __eql = key_equal(),
 			 const allocator_type& __a = allocator_type())
Index: include/bits/unordered_set.h
===================================================================
--- include/bits/unordered_set.h	(revision 213780)
+++ include/bits/unordered_set.h	(working copy)
@@ -121,15 +121,19 @@
       //@}
 
       // construct/destroy/copy
+
+      /// Default constructor.
+      unordered_set() = default;
+
       /**
        *  @brief  Default constructor creates no elements.
-       *  @param __n  Initial number of buckets.
+       *  @param __n  Minimal initial number of buckets.
        *  @param __hf  A hash functor.
        *  @param __eql  A key equality functor.
        *  @param __a  An allocator object.
        */
       explicit
-      unordered_set(size_type __n = 10,
+      unordered_set(size_type __n,
 		    const hasher& __hf = hasher(),
 		    const key_equal& __eql = key_equal(),
 		    const allocator_type& __a = allocator_type())
@@ -756,15 +760,19 @@
       //@}
 
       // construct/destroy/copy
+
+      /// Default constructor.
+      unordered_multiset() = default;
+
       /**
        *  @brief  Default constructor creates no elements.
-       *  @param __n  Initial number of buckets.
+       *  @param __n  Minimal initial number of buckets.
        *  @param __hf  A hash functor.
        *  @param __eql  A key equality functor.
        *  @param __a  An allocator object.
        */
       explicit
-      unordered_multiset(size_type __n = 10,
+      unordered_multiset(size_type __n,
 			 const hasher& __hf = hasher(),
 			 const key_equal& __eql = key_equal(),
 			 const allocator_type& __a = allocator_type())
Index: include/debug/unordered_map
===================================================================
--- include/debug/unordered_map	(revision 213780)
+++ include/debug/unordered_map	(working copy)
@@ -82,8 +82,10 @@
       typedef __gnu_debug::_Safe_local_iterator<
 	_Base_const_local_iterator, unordered_map>	const_local_iterator;
 
+      unordered_map() = default;
+
       explicit
-      unordered_map(size_type __n = 10,
+      unordered_map(size_type __n,
 		    const hasher& __hf = hasher(),
 		    const key_equal& __eql = key_equal(),
 		    const allocator_type& __a = allocator_type())
@@ -495,8 +497,10 @@
       typedef __gnu_debug::_Safe_local_iterator<
 	_Base_const_local_iterator, unordered_multimap> const_local_iterator;
 
+      unordered_multimap() = default;
+
       explicit
-      unordered_multimap(size_type __n = 10,
+      unordered_multimap(size_type __n,
 			 const hasher& __hf = hasher(),
 			 const key_equal& __eql = key_equal(),
 			 const allocator_type& __a = allocator_type())
Index: include/debug/unordered_set
===================================================================
--- include/debug/unordered_set	(revision 213780)
+++ include/debug/unordered_set	(working copy)
@@ -82,8 +82,10 @@
       typedef __gnu_debug::_Safe_local_iterator<
 	_Base_const_local_iterator, unordered_set>	const_local_iterator;
 
+      unordered_set() = default;
+
       explicit
-      unordered_set(size_type __n = 10,
+      unordered_set(size_type __n,
 		    const hasher& __hf = hasher(),
 		    const key_equal& __eql = key_equal(),
 		    const allocator_type& __a = allocator_type())
@@ -491,8 +493,10 @@
       typedef __gnu_debug::_Safe_local_iterator<
 	_Base_const_local_iterator, unordered_multiset> const_local_iterator;
 
+      unordered_multiset() = default;
+
       explicit
-      unordered_multiset(size_type __n = 10,
+      unordered_multiset(size_type __n,
 			 const hasher& __hf = hasher(),
 			 const key_equal& __eql = key_equal(),
 			 const allocator_type& __a = allocator_type())
Index: testsuite/23_containers/unordered_map/allocator/noexcept.cc
===================================================================
--- testsuite/23_containers/unordered_map/allocator/noexcept.cc	(revision 213780)
+++ testsuite/23_containers/unordered_map/allocator/noexcept.cc	(working copy)
@@ -76,3 +76,10 @@
   static_assert( noexcept( v1 = std::move(v2) ), "Move assign cannot throw" );
   static_assert( !noexcept( v1.swap(v2) ), "Swap can throw" );
 }
+
+void test04()
+{
+  typedef std::unordered_map<int, int> test_type;
+  static_assert( noexcept( test_type() ), "Default constructor do not throw" );
+  static_assert( noexcept( test_type(test_type()) ), "Move constructor do not throw" );
+}
Index: testsuite/23_containers/unordered_multimap/allocator/noexcept.cc
===================================================================
--- testsuite/23_containers/unordered_multimap/allocator/noexcept.cc	(revision 213780)
+++ testsuite/23_containers/unordered_multimap/allocator/noexcept.cc	(working copy)
@@ -76,3 +76,10 @@
   static_assert( noexcept( v1 = std::move(v2) ), "Move assign cannot throw" );
   static_assert( !noexcept( v1.swap(v2) ), "Swap can throw" );
 }
+
+void test04()
+{
+  typedef std::unordered_multimap<int, int> test_type;
+  static_assert( noexcept( test_type() ), "Default constructor do not throw" );
+  static_assert( noexcept( test_type(test_type()) ), "Move constructor do not throw" );
+}
Index: testsuite/23_containers/unordered_multiset/allocator/noexcept.cc
===================================================================
--- testsuite/23_containers/unordered_multiset/allocator/noexcept.cc	(revision 213780)
+++ testsuite/23_containers/unordered_multiset/allocator/noexcept.cc	(working copy)
@@ -76,3 +76,10 @@
   static_assert( noexcept( v1 = std::move(v2) ), "Move assign cannot throw" );
   static_assert( !noexcept( v1.swap(v2) ), "Swap can throw" );
 }
+
+void test04()
+{
+  typedef std::unordered_multiset<int> test_type;
+  static_assert( noexcept( test_type() ), "Default constructor do not throw" );
+  static_assert( noexcept( test_type(test_type()) ), "Move constructor do not throw" );
+}
Index: testsuite/23_containers/unordered_set/allocator/noexcept.cc
===================================================================
--- testsuite/23_containers/unordered_set/allocator/noexcept.cc	(revision 213780)
+++ testsuite/23_containers/unordered_set/allocator/noexcept.cc	(working copy)
@@ -76,3 +76,10 @@
   static_assert( noexcept( v1 = std::move(v2) ), "Move assign cannot throw" );
   static_assert( !noexcept( v1.swap(v2) ), "Swap can throw" );
 }
+
+void test04()
+{
+  typedef std::unordered_set<int> test_type;
+  static_assert( noexcept( test_type() ), "Default constructor do not throw" );
+  static_assert( noexcept( test_type(test_type()) ), "Move constructor do not throw" );
+}

