This patch implements http://wg21.link/p0031 which adds constexpr to
most operations on std::reverse_iterator, std::move_iterator,
std::array, as well as std::advance, std::distance, and the
range-access functions std::begin, std::rbegin etc.

Strictly speaking, those functions should only be constexpr in C++17
and not before, but this patch makes them constexpr whenever possible.
That means the changes are fully implemented for C++14 (and the
feature-test macro is defined) but only partially implemented for
C++11, because some of the functions can't be constexpr in C++11.

My thinking is that if the committee has decided that these functions
*should* be constexpr, and changed them for C++17, then it doesn't
serve users to make them non-constexpr in C++11 and C++14 just because
the standard says so.

How do other people feel about that?

The alternative would be to define a new _GLIBCXX17_CONSTEXPR macro
and use it in all these places, so they're only constexpr in C++17
(and probably for -std=gnu++14 too, but not -std=c++14).

How strict do we want to be about obeying the "implementors can't add
constexpr" rule in these cases?

Here's the patch, but I'm not committing it yet.

        * include/bits/range_access.h (begin, end, rbegin, rend, crbegin)
        (crend): Add constexpr specifier as per P0031R0.
        * include/bits/stl_iterator.h (reverse_iterator, move_iterator)
        (__make_reverse_iterator, make_reverse_iterator, make_move_iterator):
        Add _GLIBCXX_CONSTEXPR or _GLIBCXX14_CONSTEXPR as appropriate.
        * include/bits/stl_iterator_base_funcs.h (__distance, distance)
        (__advance, advance, next, prev): Likewise.
        * include/std/array (array::begin, array::end, array::rbegin)
        (array::rend, array::cbegin, array:cend, array::crbegin)
        (array::crend, array::operator[], array::at, array::front)
        (array::back, array::data): Likewise.
        * testsuite/24_iterators/headers/iterator/range_access.cc: Add
        constexpr.
        * testsuite/24_iterators/headers/iterator/synopsis.cc: Add constexpr
        to reverse_iterator operations, advance, and distance.
commit 7e46198cc511bcc8876ab9d3f11e938dbe4cfea0
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Fri Jul 15 22:31:16 2016 +0100

    Add constexpr to iterator adaptors, array and range access
    
        * include/bits/range_access.h (begin, end, rbegin, rend, crbegin)
        (crend): Add constexpr specifier as per P0031R0.
        * include/bits/stl_iterator.h (reverse_iterator, move_iterator)
        (__make_reverse_iterator, make_reverse_iterator, make_move_iterator):
        Add _GLIBCXX_CONSTEXPR or _GLIBCXX14_CONSTEXPR as appropriate.
        * include/bits/stl_iterator_base_funcs.h (__distance, distance)
        (__advance, advance, next, prev): Likewise.
        * include/std/array (array::begin, array::end, array::rbegin)
        (array::rend, array::cbegin, array:cend, array::crbegin)
        (array::crend, array::operator[], array::at, array::front)
        (array::back, array::data): Likewise.
        * testsuite/24_iterators/headers/iterator/range_access.cc: Add
        constexpr.
        * testsuite/24_iterators/headers/iterator/synopsis.cc: Add constexpr
        to reverse_iterator operations, advance, and distance.

diff --git a/libstdc++-v3/include/bits/range_access.h 
b/libstdc++-v3/include/bits/range_access.h
index e2ec072..257122e 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -44,7 +44,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     begin(_Container& __cont) -> decltype(__cont.begin())
     { return __cont.begin(); }
 
@@ -54,7 +54,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     begin(const _Container& __cont) -> decltype(__cont.begin())
     { return __cont.begin(); }
 
@@ -64,7 +64,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     end(_Container& __cont) -> decltype(__cont.end())
     { return __cont.end(); }
 
@@ -74,7 +74,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     end(const _Container& __cont) -> decltype(__cont.end())
     { return __cont.end(); }
 
@@ -83,7 +83,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __arr  Array.
    */
   template<typename _Tp, size_t _Nm>
-    inline _GLIBCXX14_CONSTEXPR _Tp*
+    constexpr _Tp*
     begin(_Tp (&__arr)[_Nm])
     { return __arr; }
 
@@ -93,7 +93,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __arr  Array.
    */
   template<typename _Tp, size_t _Nm>
-    inline _GLIBCXX14_CONSTEXPR _Tp*
+    constexpr _Tp*
     end(_Tp (&__arr)[_Nm])
     { return __arr + _Nm; }
 
@@ -112,7 +112,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline constexpr auto
+    constexpr auto
     cbegin(const _Container& __cont) noexcept(noexcept(std::begin(__cont)))
       -> decltype(std::begin(__cont))
     { return std::begin(__cont); }
@@ -123,7 +123,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline constexpr auto
+    constexpr auto
     cend(const _Container& __cont) noexcept(noexcept(std::end(__cont)))
       -> decltype(std::end(__cont))
     { return std::end(__cont); }
@@ -134,7 +134,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     rbegin(_Container& __cont) -> decltype(__cont.rbegin())
     { return __cont.rbegin(); }
 
@@ -144,7 +144,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     rbegin(const _Container& __cont) -> decltype(__cont.rbegin())
     { return __cont.rbegin(); }
 
@@ -154,7 +154,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     rend(_Container& __cont) -> decltype(__cont.rend())
     { return __cont.rend(); }
 
@@ -164,7 +164,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     rend(const _Container& __cont) -> decltype(__cont.rend())
     { return __cont.rend(); }
 
@@ -174,7 +174,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __arr  Array.
    */
   template<typename _Tp, size_t _Nm>
-    inline reverse_iterator<_Tp*>
+    constexpr reverse_iterator<_Tp*>
     rbegin(_Tp (&__arr)[_Nm])
     { return reverse_iterator<_Tp*>(__arr + _Nm); }
 
@@ -184,7 +184,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __arr  Array.
    */
   template<typename _Tp, size_t _Nm>
-    inline reverse_iterator<_Tp*>
+    constexpr reverse_iterator<_Tp*>
     rend(_Tp (&__arr)[_Nm])
     { return reverse_iterator<_Tp*>(__arr); }
 
@@ -194,7 +194,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __il  initializer_list.
    */
   template<typename _Tp>
-    inline reverse_iterator<const _Tp*>
+    constexpr reverse_iterator<const _Tp*>
     rbegin(initializer_list<_Tp> __il)
     { return reverse_iterator<const _Tp*>(__il.end()); }
 
@@ -204,7 +204,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __il  initializer_list.
    */
   template<typename _Tp>
-    inline reverse_iterator<const _Tp*>
+    constexpr reverse_iterator<const _Tp*>
     rend(initializer_list<_Tp> __il)
     { return reverse_iterator<const _Tp*>(__il.begin()); }
 
@@ -214,7 +214,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     crbegin(const _Container& __cont) -> decltype(std::rbegin(__cont))
     { return std::rbegin(__cont); }
 
@@ -224,7 +224,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @param  __cont  Container.
    */
   template<typename _Container>
-    inline auto
+    constexpr auto
     crend(const _Container& __cont) -> decltype(std::rend(__cont))
     { return std::rend(__cont); }
 
diff --git a/libstdc++-v3/include/bits/stl_iterator.h 
b/libstdc++-v3/include/bits/stl_iterator.h
index 3401cd0..4175520 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -65,6 +65,11 @@
 #include <bits/move.h>
 #include <bits/ptr_traits.h>
 
+#if __cplusplus >= 201402L
+// Full support for this feature depends on C++14 constexpr.
+# define __cpp_lib_array_constexpr 201603
+#endif
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -118,17 +123,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       */
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // 235 No specification of default ctor for reverse_iterator
+      _GLIBCXX_CONSTEXPR
       reverse_iterator() : current() { }
 
       /**
        *  This %iterator will move in the opposite direction that @p x does.
       */
-      explicit
+      explicit _GLIBCXX_CONSTEXPR
       reverse_iterator(iterator_type __x) : current(__x) { }
 
       /**
        *  The copy constructor is normal.
       */
+      _GLIBCXX_CONSTEXPR
       reverse_iterator(const reverse_iterator& __x)
       : current(__x.current) { }
 
@@ -137,13 +144,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  underlying %iterator can be converted to the type of @c current.
       */
       template<typename _Iter>
+       _GLIBCXX_CONSTEXPR
         reverse_iterator(const reverse_iterator<_Iter>& __x)
        : current(__x.base()) { }
 
       /**
        *  @return  @c current, the %iterator used for underlying work.
       */
-      iterator_type
+      _GLIBCXX_CONSTEXPR iterator_type
       base() const
       { return current; }
 
@@ -157,7 +165,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *           @c *x remains valid after @c x has been modified or
        *           destroyed. This is a bug: http://gcc.gnu.org/PR51823
       */
-      reference
+      _GLIBCXX14_CONSTEXPR reference
       operator*() const
       {
        _Iterator __tmp = current;
@@ -169,7 +177,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  This requires that @c --current is dereferenceable.
       */
-      pointer
+      _GLIBCXX14_CONSTEXPR pointer
       operator->() const
       { return &(operator*()); }
 
@@ -178,7 +186,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  Decrements the underlying iterator.
       */
-      reverse_iterator&
+      _GLIBCXX14_CONSTEXPR reverse_iterator&
       operator++()
       {
        --current;
@@ -190,7 +198,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  Decrements the underlying iterator.
       */
-      reverse_iterator
+      _GLIBCXX14_CONSTEXPR reverse_iterator
       operator++(int)
       {
        reverse_iterator __tmp = *this;
@@ -203,7 +211,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  Increments the underlying iterator.
       */
-      reverse_iterator&
+      _GLIBCXX14_CONSTEXPR reverse_iterator&
       operator--()
       {
        ++current;
@@ -215,7 +223,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  Increments the underlying iterator.
       */
-      reverse_iterator
+      _GLIBCXX14_CONSTEXPR reverse_iterator
       operator--(int)
       {
        reverse_iterator __tmp = *this;
@@ -228,7 +236,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  The underlying iterator must be a Random Access Iterator.
       */
-      reverse_iterator
+      _GLIBCXX_CONSTEXPR reverse_iterator
       operator+(difference_type __n) const
       { return reverse_iterator(current - __n); }
 
@@ -238,7 +246,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  Moves the underlying iterator backwards @a __n steps.
        *  The underlying iterator must be a Random Access Iterator.
       */
-      reverse_iterator&
+      _GLIBCXX14_CONSTEXPR reverse_iterator&
       operator+=(difference_type __n)
       {
        current -= __n;
@@ -250,7 +258,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  The underlying iterator must be a Random Access Iterator.
       */
-      reverse_iterator
+      _GLIBCXX_CONSTEXPR reverse_iterator
       operator-(difference_type __n) const
       { return reverse_iterator(current + __n); }
 
@@ -260,7 +268,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *  Moves the underlying iterator forwards @a __n steps.
        *  The underlying iterator must be a Random Access Iterator.
       */
-      reverse_iterator&
+      _GLIBCXX14_CONSTEXPR reverse_iterator&
       operator-=(difference_type __n)
       {
        current += __n;
@@ -272,7 +280,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        *
        *  The underlying iterator must be a Random Access Iterator.
       */
-      reference
+      _GLIBCXX14_CONSTEXPR reference
       operator[](difference_type __n) const
       { return *(*this + __n); }
     };
@@ -288,49 +296,50 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *
   */
   template<typename _Iterator>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator==(const reverse_iterator<_Iterator>& __x,
               const reverse_iterator<_Iterator>& __y)
     { return __x.base() == __y.base(); }
 
   template<typename _Iterator>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator<(const reverse_iterator<_Iterator>& __x,
              const reverse_iterator<_Iterator>& __y)
     { return __y.base() < __x.base(); }
 
   template<typename _Iterator>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator!=(const reverse_iterator<_Iterator>& __x,
               const reverse_iterator<_Iterator>& __y)
     { return !(__x == __y); }
 
   template<typename _Iterator>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator>(const reverse_iterator<_Iterator>& __x,
              const reverse_iterator<_Iterator>& __y)
     { return __y < __x; }
 
   template<typename _Iterator>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator<=(const reverse_iterator<_Iterator>& __x,
               const reverse_iterator<_Iterator>& __y)
     { return !(__y < __x); }
 
   template<typename _Iterator>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator>=(const reverse_iterator<_Iterator>& __x,
               const reverse_iterator<_Iterator>& __y)
     { return !(__x < __y); }
 
   template<typename _Iterator>
-    inline typename reverse_iterator<_Iterator>::difference_type
+    inline _GLIBCXX_CONSTEXPR
+    typename reverse_iterator<_Iterator>::difference_type
     operator-(const reverse_iterator<_Iterator>& __x,
              const reverse_iterator<_Iterator>& __y)
     { return __y.base() - __x.base(); }
 
   template<typename _Iterator>
-    inline reverse_iterator<_Iterator>
+    inline _GLIBCXX_CONSTEXPR reverse_iterator<_Iterator>
     operator+(typename reverse_iterator<_Iterator>::difference_type __n,
              const reverse_iterator<_Iterator>& __x)
     { return reverse_iterator<_Iterator>(__x.base() - __n); }
@@ -338,37 +347,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // _GLIBCXX_RESOLVE_LIB_DEFECTS
   // DR 280. Comparison of reverse_iterator to const reverse_iterator.
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator==(const reverse_iterator<_IteratorL>& __x,
               const reverse_iterator<_IteratorR>& __y)
     { return __x.base() == __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator<(const reverse_iterator<_IteratorL>& __x,
              const reverse_iterator<_IteratorR>& __y)
     { return __y.base() < __x.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator!=(const reverse_iterator<_IteratorL>& __x,
               const reverse_iterator<_IteratorR>& __y)
     { return !(__x == __y); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator>(const reverse_iterator<_IteratorL>& __x,
              const reverse_iterator<_IteratorR>& __y)
     { return __y < __x; }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator<=(const reverse_iterator<_IteratorL>& __x,
               const reverse_iterator<_IteratorR>& __y)
     { return !(__y < __x); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    inline _GLIBCXX_CONSTEXPR bool
     operator>=(const reverse_iterator<_IteratorL>& __x,
               const reverse_iterator<_IteratorR>& __y)
     { return !(__x < __y); }
@@ -376,7 +385,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _IteratorL, typename _IteratorR>
 #if __cplusplus >= 201103L
     // DR 685.
-    inline auto
+    constexpr auto
     operator-(const reverse_iterator<_IteratorL>& __x,
              const reverse_iterator<_IteratorR>& __y)
     -> decltype(__y.base() - __x.base())
@@ -391,7 +400,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 201103L
   // Same as C++14 make_reverse_iterator but used in C++03 mode too.
   template<typename _Iterator>
-    inline reverse_iterator<_Iterator>
+    inline _GLIBCXX_CONSTEXPR reverse_iterator<_Iterator>
     __make_reverse_iterator(_Iterator __i)
     { return reverse_iterator<_Iterator>(__i); }
 
@@ -402,7 +411,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // DR 2285. make_reverse_iterator
   /// Generator function for reverse_iterator.
   template<typename _Iterator>
-    inline reverse_iterator<_Iterator>
+    constexpr reverse_iterator<_Iterator>
     make_reverse_iterator(_Iterator __i)
     { return reverse_iterator<_Iterator>(__i); }
 # endif
@@ -1018,37 +1027,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                         typename remove_reference<__base_ref>::type&&,
                         __base_ref>::type              reference;
 
+      constexpr
       move_iterator()
       : _M_current() { }
 
-      explicit
+      explicit constexpr
       move_iterator(iterator_type __i)
       : _M_current(__i) { }
 
       template<typename _Iter>
+       constexpr
        move_iterator(const move_iterator<_Iter>& __i)
        : _M_current(__i.base()) { }
 
-      iterator_type
+      constexpr iterator_type
       base() const
       { return _M_current; }
 
-      reference
+      constexpr reference
       operator*() const
       { return static_cast<reference>(*_M_current); }
 
-      pointer
+      constexpr pointer
       operator->() const
       { return _M_current; }
 
-      move_iterator&
+      _GLIBCXX14_CONSTEXPR move_iterator&
       operator++()
       {
        ++_M_current;
        return *this;
       }
 
-      move_iterator
+      _GLIBCXX14_CONSTEXPR move_iterator
       operator++(int)
       {
        move_iterator __tmp = *this;
@@ -1056,14 +1067,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        return __tmp;
       }
 
-      move_iterator&
+      _GLIBCXX14_CONSTEXPR move_iterator&
       operator--()
       {
        --_M_current;
        return *this;
       }
 
-      move_iterator
+      _GLIBCXX14_CONSTEXPR move_iterator
       operator--(int)
       {
        move_iterator __tmp = *this;
@@ -1071,29 +1082,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        return __tmp;
       }
 
-      move_iterator
+      constexpr move_iterator
       operator+(difference_type __n) const
       { return move_iterator(_M_current + __n); }
 
-      move_iterator&
+      _GLIBCXX14_CONSTEXPR move_iterator&
       operator+=(difference_type __n)
       {
        _M_current += __n;
        return *this;
       }
 
-      move_iterator
+      constexpr move_iterator
       operator-(difference_type __n) const
       { return move_iterator(_M_current - __n); }
     
-      move_iterator&
+      _GLIBCXX14_CONSTEXPR move_iterator&
       operator-=(difference_type __n)
       { 
        _M_current -= __n;
        return *this;
       }
 
-      reference
+      constexpr reference
       operator[](difference_type __n) const
       { return std::move(_M_current[__n]); }
     };
@@ -1102,100 +1113,100 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // why there are always 2 versions for most of the move_iterator
   // operators.
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    constexpr bool
     operator==(const move_iterator<_IteratorL>& __x,
               const move_iterator<_IteratorR>& __y)
     { return __x.base() == __y.base(); }
 
   template<typename _Iterator>
-    inline bool
+    constexpr bool
     operator==(const move_iterator<_Iterator>& __x,
               const move_iterator<_Iterator>& __y)
     { return __x.base() == __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    constexpr bool
     operator!=(const move_iterator<_IteratorL>& __x,
               const move_iterator<_IteratorR>& __y)
     { return !(__x == __y); }
 
   template<typename _Iterator>
-    inline bool
+    constexpr bool
     operator!=(const move_iterator<_Iterator>& __x,
               const move_iterator<_Iterator>& __y)
     { return !(__x == __y); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    constexpr bool
     operator<(const move_iterator<_IteratorL>& __x,
              const move_iterator<_IteratorR>& __y)
     { return __x.base() < __y.base(); }
 
   template<typename _Iterator>
-    inline bool
+    constexpr bool
     operator<(const move_iterator<_Iterator>& __x,
              const move_iterator<_Iterator>& __y)
     { return __x.base() < __y.base(); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    constexpr bool
     operator<=(const move_iterator<_IteratorL>& __x,
               const move_iterator<_IteratorR>& __y)
     { return !(__y < __x); }
 
   template<typename _Iterator>
-    inline bool
+    constexpr bool
     operator<=(const move_iterator<_Iterator>& __x,
               const move_iterator<_Iterator>& __y)
     { return !(__y < __x); }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    constexpr bool
     operator>(const move_iterator<_IteratorL>& __x,
              const move_iterator<_IteratorR>& __y)
     { return __y < __x; }
 
   template<typename _Iterator>
-    inline bool
+    constexpr bool
     operator>(const move_iterator<_Iterator>& __x,
              const move_iterator<_Iterator>& __y)
     { return __y < __x; }
 
   template<typename _IteratorL, typename _IteratorR>
-    inline bool
+    constexpr bool
     operator>=(const move_iterator<_IteratorL>& __x,
               const move_iterator<_IteratorR>& __y)
     { return !(__x < __y); }
 
   template<typename _Iterator>
-    inline bool
+    constexpr bool
     operator>=(const move_iterator<_Iterator>& __x,
               const move_iterator<_Iterator>& __y)
     { return !(__x < __y); }
 
   // DR 685.
   template<typename _IteratorL, typename _IteratorR>
-    inline auto
+    constexpr auto
     operator-(const move_iterator<_IteratorL>& __x,
              const move_iterator<_IteratorR>& __y)
     -> decltype(__x.base() - __y.base())
     { return __x.base() - __y.base(); }
 
   template<typename _Iterator>
-    inline auto
+    constexpr auto
     operator-(const move_iterator<_Iterator>& __x,
              const move_iterator<_Iterator>& __y)
     -> decltype(__x.base() - __y.base())
     { return __x.base() - __y.base(); }
 
   template<typename _Iterator>
-    inline move_iterator<_Iterator>
+    constexpr move_iterator<_Iterator>
     operator+(typename move_iterator<_Iterator>::difference_type __n,
              const move_iterator<_Iterator>& __x)
     { return __x + __n; }
 
   template<typename _Iterator>
-    inline move_iterator<_Iterator>
+    constexpr move_iterator<_Iterator>
     make_move_iterator(_Iterator __i)
     { return move_iterator<_Iterator>(__i); }
 
@@ -1203,7 +1214,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     = typename conditional<__move_if_noexcept_cond
       <typename iterator_traits<_Iterator>::value_type>::value,
                 _Iterator, move_iterator<_Iterator>>::type>
-    inline _ReturnType
+    constexpr _ReturnType
     __make_move_if_noexcept_iterator(_Iterator __i)
     { return _ReturnType(__i); }
 
@@ -1212,7 +1223,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp, typename _ReturnType
     = typename conditional<__move_if_noexcept_cond<_Tp>::value,
                           const _Tp*, move_iterator<_Tp*>>::type>
-    inline _ReturnType
+    constexpr _ReturnType
     __make_move_if_noexcept_iterator(_Tp* __i)
     { return _ReturnType(__i); }
 
diff --git a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h 
b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
index a8b156d..a8ce4ed 100644
--- a/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
+++ b/libstdc++-v3/include/bits/stl_iterator_base_funcs.h
@@ -75,7 +75,8 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   template<typename _InputIterator>
-    inline typename iterator_traits<_InputIterator>::difference_type
+    inline _GLIBCXX14_CONSTEXPR
+    typename iterator_traits<_InputIterator>::difference_type
     __distance(_InputIterator __first, _InputIterator __last,
                input_iterator_tag)
     {
@@ -92,7 +93,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _RandomAccessIterator>
-    inline typename iterator_traits<_RandomAccessIterator>::difference_type
+    inline _GLIBCXX14_CONSTEXPR
+    typename iterator_traits<_RandomAccessIterator>::difference_type
     __distance(_RandomAccessIterator __first, _RandomAccessIterator __last,
                random_access_iterator_tag)
     {
@@ -131,7 +133,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  and are constant time.  For other %iterator classes they are linear time.
   */
   template<typename _InputIterator>
-    inline typename iterator_traits<_InputIterator>::difference_type
+    inline _GLIBCXX14_CONSTEXPR
+    typename iterator_traits<_InputIterator>::difference_type
     distance(_InputIterator __first, _InputIterator __last)
     {
       // concept requirements -- taken care of in __distance
@@ -140,7 +143,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _InputIterator, typename _Distance>
-    inline void
+    inline _GLIBCXX14_CONSTEXPR void
     __advance(_InputIterator& __i, _Distance __n, input_iterator_tag)
     {
       // concept requirements
@@ -151,7 +154,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _BidirectionalIterator, typename _Distance>
-    inline void
+    inline _GLIBCXX14_CONSTEXPR void
     __advance(_BidirectionalIterator& __i, _Distance __n,
              bidirectional_iterator_tag)
     {
@@ -167,7 +170,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _RandomAccessIterator, typename _Distance>
-    inline void
+    inline _GLIBCXX14_CONSTEXPR void
     __advance(_RandomAccessIterator& __i, _Distance __n,
               random_access_iterator_tag)
     {
@@ -190,7 +193,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  and are constant time.  For other %iterator classes they are linear time.
   */
   template<typename _InputIterator, typename _Distance>
-    inline void
+    inline _GLIBCXX14_CONSTEXPR void
     advance(_InputIterator& __i, _Distance __n)
     {
       // concept requirements -- taken care of in __advance
@@ -201,7 +204,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #if __cplusplus >= 201103L
 
   template<typename _ForwardIterator>
-    inline _ForwardIterator
+    inline _GLIBCXX14_CONSTEXPR _ForwardIterator
     next(_ForwardIterator __x, typename
         iterator_traits<_ForwardIterator>::difference_type __n = 1)
     {
@@ -213,7 +216,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
   template<typename _BidirectionalIterator>
-    inline _BidirectionalIterator
+    inline _GLIBCXX14_CONSTEXPR _BidirectionalIterator
     prev(_BidirectionalIterator __x, typename
         iterator_traits<_BidirectionalIterator>::difference_type __n = 1) 
     {
diff --git a/libstdc++-v3/include/std/array b/libstdc++-v3/include/std/array
index 73d2669..00b18bb 100644
--- a/libstdc++-v3/include/std/array
+++ b/libstdc++-v3/include/std/array
@@ -122,51 +122,51 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       { std::swap_ranges(begin(), end(), __other.begin()); }
 
       // Iterators.
-      iterator
+      _GLIBCXX14_CONSTEXPR iterator
       begin() noexcept
       { return iterator(data()); }
 
-      const_iterator
+      constexpr const_iterator
       begin() const noexcept
       { return const_iterator(data()); }
 
-      iterator
+      _GLIBCXX14_CONSTEXPR iterator
       end() noexcept
       { return iterator(data() + _Nm); }
 
-      const_iterator
+      constexpr const_iterator
       end() const noexcept
       { return const_iterator(data() + _Nm); }
 
-      reverse_iterator
+      _GLIBCXX14_CONSTEXPR reverse_iterator
       rbegin() noexcept
       { return reverse_iterator(end()); }
 
-      const_reverse_iterator
+      constexpr const_reverse_iterator
       rbegin() const noexcept
       { return const_reverse_iterator(end()); }
 
-      reverse_iterator
+      _GLIBCXX14_CONSTEXPR reverse_iterator
       rend() noexcept
       { return reverse_iterator(begin()); }
 
-      const_reverse_iterator
+      constexpr const_reverse_iterator
       rend() const noexcept
       { return const_reverse_iterator(begin()); }
 
-      const_iterator
+      constexpr const_iterator
       cbegin() const noexcept
       { return const_iterator(data()); }
 
-      const_iterator
+      constexpr const_iterator
       cend() const noexcept
       { return const_iterator(data() + _Nm); }
 
-      const_reverse_iterator
+      constexpr const_reverse_iterator
       crbegin() const noexcept
       { return const_reverse_iterator(end()); }
 
-      const_reverse_iterator
+      constexpr const_reverse_iterator
       crend() const noexcept
       { return const_reverse_iterator(begin()); }
 
@@ -181,7 +181,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       empty() const noexcept { return size() == 0; }
 
       // Element access.
-      reference
+      _GLIBCXX14_CONSTEXPR reference
       operator[](size_type __n) noexcept
       { return _AT_Type::_S_ref(_M_elems, __n); }
 
@@ -189,7 +189,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       operator[](size_type __n) const noexcept
       { return _AT_Type::_S_ref(_M_elems, __n); }
 
-      reference
+      _GLIBCXX14_CONSTEXPR reference
       at(size_type __n)
       {
        if (__n >= _Nm)
@@ -211,7 +211,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
             _AT_Type::_S_ref(_M_elems, 0));
       }
 
-      reference
+      _GLIBCXX14_CONSTEXPR reference
       front() noexcept
       { return *begin(); }
 
@@ -219,7 +219,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       front() const noexcept
       { return _AT_Type::_S_ref(_M_elems, 0); }
 
-      reference
+      _GLIBCXX14_CONSTEXPR reference
       back() noexcept
       { return _Nm ? *(end() - 1) : *end(); }
 
@@ -230,11 +230,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
                   : _AT_Type::_S_ref(_M_elems, 0);
       }
 
-      pointer
+      _GLIBCXX14_CONSTEXPR pointer
       data() noexcept
       { return _AT_Type::_S_ptr(_M_elems); }
 
-      const_pointer
+      constexpr const_pointer
       data() const noexcept
       { return _AT_Type::_S_ptr(_M_elems); }
     };
diff --git 
a/libstdc++-v3/testsuite/24_iterators/headers/iterator/range_access.cc 
b/libstdc++-v3/testsuite/24_iterators/headers/iterator/range_access.cc
index b66381e..e40cfc7 100644
--- a/libstdc++-v3/testsuite/24_iterators/headers/iterator/range_access.cc
+++ b/libstdc++-v3/testsuite/24_iterators/headers/iterator/range_access.cc
@@ -22,12 +22,12 @@
 
 namespace std
 {
-  template<class C> auto begin(C& c) -> decltype(c.begin());
-  template<class C> auto begin(const C& c) -> decltype(c.begin());
+  template<class C> constexpr auto begin(C& c) -> decltype(c.begin());
+  template<class C> constexpr auto begin(const C& c) -> decltype(c.begin());
 
-  template<class C> auto end(C& c) -> decltype(c.end());
-  template<class C> auto end(const C& c) -> decltype(c.end());
+  template<class C> constexpr auto end(C& c) -> decltype(c.end());
+  template<class C> constexpr auto end(const C& c) -> decltype(c.end());
 
-  template<class T, size_t N> T* begin(T (&array)[N]);
-  template<class T, size_t N> T* end(T (&array)[N]);
+  template<class T, size_t N> constexpr T* begin(T (&array)[N]);
+  template<class T, size_t N> constexpr T* end(T (&array)[N]);
 }
diff --git a/libstdc++-v3/testsuite/24_iterators/headers/iterator/synopsis.cc 
b/libstdc++-v3/testsuite/24_iterators/headers/iterator/synopsis.cc
index ed01258..cd843a2 100644
--- a/libstdc++-v3/testsuite/24_iterators/headers/iterator/synopsis.cc
+++ b/libstdc++-v3/testsuite/24_iterators/headers/iterator/synopsis.cc
@@ -36,46 +36,54 @@ namespace std {
 
   // 24.3.4, iterator operations:
   template <class InputIterator, class Distance>
-  void 
+  constexpr void
   advance(InputIterator& i, Distance n);
 
   template <class InputIterator>
-  typename iterator_traits<InputIterator>::difference_type
+  constexpr typename iterator_traits<InputIterator>::difference_type
   distance(InputIterator first, InputIterator last);
 
   // 24.4, predefined iterators:
   template <class Iterator> class reverse_iterator;
 
   template <class Iterator>
+  constexpr
   bool operator==(const reverse_iterator<Iterator>& x,
                  const reverse_iterator<Iterator>& y);
 
   template <class Iterator>
+  constexpr
   bool operator<(const reverse_iterator<Iterator>& x,
                 const reverse_iterator<Iterator>& y);
 
   template <class Iterator>
+  constexpr
   bool operator!=(const reverse_iterator<Iterator>& x,
                  const reverse_iterator<Iterator>& y);
 
   template <class Iterator>
+  constexpr
   bool operator>(const reverse_iterator<Iterator>& x,
                 const reverse_iterator<Iterator>& y);
 
   template <class Iterator>
+  constexpr
   bool operator>=(const reverse_iterator<Iterator>& x,
                  const reverse_iterator<Iterator>& y);
 
   template <class Iterator>
+  constexpr
   bool operator<=(const reverse_iterator<Iterator>& x,
                  const reverse_iterator<Iterator>& y);
 
   template <class Iterator>
+  constexpr
   typename reverse_iterator<Iterator>::difference_type 
   operator-(const reverse_iterator<Iterator>& x, 
            const reverse_iterator<Iterator>& y);
 
   template <class Iterator>
+  constexpr
   reverse_iterator<Iterator>
   operator+(typename reverse_iterator<Iterator>::difference_type n,
            const reverse_iterator<Iterator>& x);

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