libstdc++: integer-class types as per [iterator.concept.winc]

This implements signed and unsigned integer-class types, whose width is
one bit larger than the widest supported signed and unsigned integral
type respectively.  In our case this is either __int128 and unsigned
__int128, or long long and unsigned long long.

Internally, the two integer-class types are represented as a largest
supported unsigned integral type plus one extra bit.  The signed
integer-class type is represented in two's complement form with the
extra bit acting as the sign bit.

libstdc++-v3/ChangeLog:

	* include/Makefile.am (bits_headers): Add new header
	<bits/max_size_type.h>.
	* include/Makefile.in: Regenerate.
	* include/bits/iterator_concepts.h
	(ranges::__detail::__max_diff_type): Remove definition, replace
	with forward declaration of class __max_diff_type.
	(__detail::__max_size_type): Remove definition, replace with
	forward declaration of class __max_size_type.
	(__detail::__is_unsigned_int128, __is_signed_int128)
	(__is_int128): New concepts.
	(__detail::__is_integer_like): Accept __int128 and unsigned
	__int128.
	(__detail::__is_signed_integer_like): Accept __int128.
	* include/bits/max_size_type.h: New header.
	* include/bits/range_access.h: Include <bits/max_size_type.h>.
	(__detail::__to_unsigned_like): Two new overloads.
	* testsuite/std/ranges/iota/difference_type.cc: New test.
	* testsuite/std/ranges/iota/max_size_type.cc: New test.

7 files changed, 1219 insertions, 4 deletions

diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index e131ce0..1dff386 100644
--- a/libstdc++-v3/include/Makefile.am
+++ b/libstdc++-v3/include/Makefile.am
@@ -143,6 +143,7 @@ bits_headers = \
 	${bits_srcdir}/locale_facets_nonio.tcc \
 	${bits_srcdir}/localefwd.h \
 	${bits_srcdir}/mask_array.h \
+	${bits_srcdir}/max_size_type.h \
 	${bits_srcdir}/memoryfwd.h \
 	${bits_srcdir}/move.h \
 	${bits_srcdir}/node_handle.h \
diff --git a/libstdc++-v3/include/Makefile.in b/libstdc++-v3/include/Makefile.in
index c0b71e1..1637101 100644
--- a/libstdc++-v3/include/Makefile.in
+++ b/libstdc++-v3/include/Makefile.in
@@ -489,6 +489,7 @@ bits_headers = \
 	${bits_srcdir}/locale_facets_nonio.tcc \
 	${bits_srcdir}/localefwd.h \
 	${bits_srcdir}/mask_array.h \
+	${bits_srcdir}/max_size_type.h \
 	${bits_srcdir}/memoryfwd.h \
 	${bits_srcdir}/move.h \
 	${bits_srcdir}/node_handle.h \
diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index aad6c2d..5033f2b 100644
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -511,12 +511,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   namespace ranges::__detail
   {
+    class __max_diff_type;
+    class __max_size_type;
+
+    template<typename _Tp>
+      concept __is_signed_int128
 #if __SIZEOF_INT128__
-    using __max_diff_type = __int128;
-    using __max_size_type = unsigned __int128;
+	= same_as<_Tp, __int128>;
 #else
-    using __max_diff_type = long long;
-    using __max_size_type = unsigned long long;
+	= false;
+#endif
+
+    template<typename _Tp>
+      concept __is_unsigned_int128
+#if __SIZEOF_INT128__
+	= same_as<_Tp, unsigned __int128>;
+#else
+	= false;
 #endif
 
     template<typename _Tp>
@@ -526,11 +537,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       concept __integral_nonbool = integral<_Tp> && !__cv_bool<_Tp>;
 
     template<typename _Tp>
+      concept __is_int128 = __is_signed_int128<_Tp> || __is_unsigned_int128<_Tp>;
+
+    template<typename _Tp>
       concept __is_integer_like = __integral_nonbool<_Tp>
+	|| __is_int128<_Tp>
 	|| same_as<_Tp, __max_diff_type> || same_as<_Tp, __max_size_type>;
 
     template<typename _Tp>
       concept __is_signed_integer_like = signed_integral<_Tp>
+	|| __is_signed_int128<_Tp>
 	|| same_as<_Tp, __max_diff_type>;
 
   } // namespace ranges::__detail
diff --git a/libstdc++-v3/include/bits/max_size_type.h b/libstdc++-v3/include/bits/max_size_type.h
new file mode 100644
index 0000000..8e61662
--- /dev/null
+++ b/libstdc++-v3/include/bits/max_size_type.h
@@ -0,0 +1,753 @@
+// <max_size_type.h> -*- C++ -*-
+
+// Copyright (C) 2019-2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file bits/max_size_type.h.
+ *  This is an internal header file, included by other library headers.
+ *  Do not attempt to use it directly. @headername{iterator}
+ */
+
+#ifndef _GLIBCXX_MAX_SIZE_TYPE_H
+#define _GLIBCXX_MAX_SIZE_TYPE_H 1
+
+#pragma GCC system_header
+
+#if __cplusplus > 201703L && __cpp_lib_concepts
+#include <ext/numeric_traits.h>
+#include <numbers>
+
+// This header implements unsigned and signed integer-class types (as per
+// [iterator.concept.winc]) that are one bit wider than the widest supported
+// integer type.
+//
+// The set of integer types we consider includes __int128 and unsigned __int128
+// (when they exist), even though they are really integer types only in GNU
+// mode.  This is to obtain a consistent ABI for these integer-class types
+// across strict mode and GNU mode.
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+template<typename _Tp>
+  struct numeric_limits;
+
+namespace ranges
+{
+  namespace __detail
+  {
+    class __max_size_type
+    {
+    public:
+      __max_size_type() = default;
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	constexpr
+	__max_size_type(_Tp __i) noexcept
+	  : _M_val(__i), _M_msb(__i < 0)
+	{ }
+
+      constexpr explicit
+      __max_size_type(const __max_diff_type& __d) noexcept;
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	constexpr explicit operator _Tp() const noexcept
+	{ return _M_val; }
+
+      constexpr explicit
+      operator bool() const noexcept
+      { return _M_val != 0 || _M_msb != 0; }
+
+      constexpr __max_size_type
+      operator+() const noexcept
+      { return *this; }
+
+      constexpr __max_size_type
+      operator~() const noexcept
+      { return __max_size_type{~_M_val, !_M_msb}; }
+
+      constexpr __max_size_type
+      operator-() const noexcept
+      { return operator~() + 1; }
+
+      constexpr __max_size_type&
+      operator+=(const __max_size_type& __r) noexcept
+      {
+	const auto __sum = _M_val + __r._M_val;
+	const bool __overflow = (__sum < _M_val);
+	_M_msb = _M_msb ^ __r._M_msb ^ __overflow;
+	_M_val = __sum;
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator-=(const __max_size_type& __r) noexcept
+      { return *this += -__r; }
+
+      constexpr __max_size_type&
+      operator*=(__max_size_type __r) noexcept
+      {
+	constexpr __max_size_type __threshold
+	  = __rep(1) << (_S_rep_bits / 2 - 1);
+	if (_M_val < __threshold && __r < __threshold)
+	  // When both operands are below this threshold then the
+	  // multiplication can be safely computed in the base precision.
+	  _M_val = _M_val * __r._M_val;
+	else
+	  {
+	    // Otherwise, perform the multiplication in four steps, by
+	    // decomposing the LHS and the RHS into 2*x+a and 2*y+b,
+	    // respectively, and computing 4*x*y + 2*x*b + 2*y*a + a*b.
+	    const bool __lsb = _M_val & 1;
+	    const bool __rlsb = __r._M_val & 1;
+	    *this >>= 1;
+	    __r >>= 1;
+	    _M_val = (2 * _M_val * __r._M_val
+		      + _M_val * __rlsb + __r._M_val * __lsb);
+	    *this <<= 1;
+	    *this += __rlsb * __lsb;
+	  }
+
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator/=(const __max_size_type& __r) noexcept
+      {
+	__glibcxx_assert(__r != 0);
+
+	if (!_M_msb && !__r._M_msb) [[likely]]
+	  _M_val /= __r._M_val;
+	else if (_M_msb && __r._M_msb)
+	  {
+	    _M_val = (_M_val >= __r._M_val);
+	    _M_msb = 0;
+	  }
+	else if (!_M_msb && __r._M_msb)
+	  _M_val = 0;
+	else if (_M_msb && !__r._M_msb)
+	  {
+	    // The non-trivial case: the dividend has its MSB set and the
+	    // divisor doesn't.  In this case we compute ((LHS/2)/RHS)*2
+	    // in the base precision.  This quantity is either the true
+	    // quotient or one less than the true quotient.
+	    const auto __orig = *this;
+	    *this >>= 1;
+	    _M_val /= __r._M_val;
+	    *this <<= 1;
+	    if (__orig - *this * __r >= __r)
+	      ++_M_val;
+	  }
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator%=(const __max_size_type& __r) noexcept
+      {
+	if (!_M_msb && !__r._M_msb) [[likely]]
+	  _M_val %= __r._M_val;
+	else
+	  *this -= (*this / __r) * __r;
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator<<=(const __max_size_type& __r) noexcept
+      {
+	__glibcxx_assert(__r <= _S_rep_bits);
+	if (__r != 0)
+	  {
+	    _M_msb = (_M_val >> (_S_rep_bits - __r._M_val)) & 1;
+
+	    if (__r._M_val == _S_rep_bits) [[unlikely]]
+	      _M_val = 0;
+	    else
+	      _M_val <<= __r._M_val;
+	  }
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator>>=(const __max_size_type& __r) noexcept
+      {
+	__glibcxx_assert(__r <= _S_rep_bits);
+	if (__r != 0)
+	  {
+	    if (__r._M_val == _S_rep_bits) [[unlikely]]
+	      _M_val = 0;
+	    else
+	      _M_val >>= __r._M_val;
+
+	    if (_M_msb) [[unlikely]]
+	      {
+		_M_val |= __rep(1) << (_S_rep_bits - __r._M_val);
+		_M_msb = 0;
+	      }
+	  }
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator&=(const __max_size_type& __r) noexcept
+      {
+	_M_val &= __r._M_val;
+	_M_msb &= __r._M_msb;
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator|=(const __max_size_type& __r) noexcept
+      {
+	_M_val |= __r._M_val;
+	_M_msb |= __r._M_msb;
+	return *this;
+      }
+
+      constexpr __max_size_type&
+      operator^=(const __max_size_type& __r) noexcept
+      {
+	_M_val ^= __r._M_val;
+	_M_msb ^= __r._M_msb;
+	return *this;
+      }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator+=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a + __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator-=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a - __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator*=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a * __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator/=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a / __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator%=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a % __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator&=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a & __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator|=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a | __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator^=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a ^ __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator<<=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a << __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator>>=(_Tp& __a, const __max_size_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a >> __b)); }
+
+      friend constexpr __max_size_type
+      operator+(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l += __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator-(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l -= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator*(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l *= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator/(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l /= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator%(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l %= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator<<(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l <<= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator>>(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l >>= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator&(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l &= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator|(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l |= __r;
+	return __l;
+      }
+
+      friend constexpr __max_size_type
+      operator^(__max_size_type __l, const __max_size_type& __r) noexcept
+      {
+	__l ^= __r;
+	return __l;
+      }
+
+      friend constexpr bool
+      operator==(const __max_size_type& __l, const __max_size_type& __r) noexcept
+      { return __l._M_val == __r._M_val && __l._M_msb == __r._M_msb; }
+
+      friend constexpr bool
+      operator!=(const __max_size_type& __l, const __max_size_type& __r) noexcept
+      { return !(__l == __r); }
+
+      friend constexpr bool
+      operator<(const __max_size_type& __l, const __max_size_type& __r) noexcept
+      {
+	if (__l._M_msb == __r._M_msb)
+	  return __l._M_val < __r._M_val;
+	else
+	  return __r._M_msb;
+      }
+
+      friend constexpr bool
+      operator>(const __max_size_type& __l, const __max_size_type& __r) noexcept
+      { return __r < __l; }
+
+      friend constexpr bool
+      operator<=(const __max_size_type& __l, const __max_size_type& __r) noexcept
+      { return !(__l > __r); }
+
+      friend constexpr bool
+      operator>=(const __max_size_type& __l, const __max_size_type& __r) noexcept
+      { return __r <= __l; }
+
+#if __SIZEOF_INT128__
+      using __rep = unsigned __int128;
+#else
+      using __rep = unsigned long long;
+#endif
+      static constexpr size_t _S_rep_bits = sizeof(__rep) * __CHAR_BIT__;
+    private:
+      __rep _M_val = 0;
+      unsigned _M_msb:1 = 0;
+
+      constexpr explicit
+      __max_size_type(__rep __val, int __msb) noexcept
+	: _M_val(__val), _M_msb(__msb)
+      { }
+
+      friend __max_diff_type;
+      friend std::numeric_limits<__max_size_type>;
+      friend std::numeric_limits<__max_diff_type>;
+    };
+
+    class __max_diff_type
+    {
+    public:
+      __max_diff_type() = default;
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	constexpr
+	__max_diff_type(_Tp __i) noexcept
+	  : _M_rep(__i)
+	{ }
+
+      constexpr explicit
+      __max_diff_type(const __max_size_type& __d) noexcept
+	: _M_rep(__d)
+      { }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	constexpr explicit operator _Tp() const noexcept
+	{ return static_cast<_Tp>(_M_rep); }
+
+      constexpr explicit
+      operator bool() const noexcept
+      { return _M_rep != 0; }
+
+      constexpr __max_diff_type
+      operator+() const noexcept
+      { return *this; }
+
+      constexpr __max_diff_type
+      operator-() const noexcept
+      { return __max_diff_type(-_M_rep); }
+
+      constexpr __max_diff_type
+      operator~() const noexcept
+      { return __max_diff_type(~_M_rep); }
+
+      constexpr __max_diff_type&
+      operator+=(const __max_diff_type& __r) noexcept
+      {
+	_M_rep += __r._M_rep;
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator-=(const __max_diff_type& __r) noexcept
+      {
+	_M_rep -= __r._M_rep;
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator*=(const __max_diff_type& __r) noexcept
+      {
+	_M_rep *= __r._M_rep;
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator/=(const __max_diff_type& __r) noexcept
+      {
+	__glibcxx_assert (__r != 0);
+	const bool __neg = *this < 0;
+	const bool __rneg = __r < 0;
+	if (!__neg && !__rneg)
+	  _M_rep = _M_rep / __r._M_rep;
+	else if (__neg && __rneg)
+	  _M_rep = -_M_rep / -__r._M_rep;
+	else if (__neg && !__rneg)
+	  _M_rep = -(-_M_rep / __r._M_rep);
+	else
+	  _M_rep = -(_M_rep / -__r._M_rep);
+	return *this ;
+      }
+
+      constexpr __max_diff_type&
+      operator%=(const __max_diff_type& __r) noexcept
+      {
+	__glibcxx_assert (__r != 0);
+	if (*this >= 0 && __r > 0)
+	  _M_rep %= __r._M_rep;
+	else
+	  *this -= (*this / __r) * __r;
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator<<=(const __max_diff_type& __r) noexcept
+      {
+	_M_rep.operator<<=(__r._M_rep);
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator>>=(const __max_diff_type& __r) noexcept
+      {
+	// Arithmetic right shift.
+	const auto __msb = _M_rep._M_msb;
+	_M_rep >>= __r._M_rep;
+	_M_rep._M_msb |= __msb;
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator&=(const __max_diff_type& __r) noexcept
+      {
+	_M_rep &= __r._M_rep;
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator|=(const __max_diff_type& __r) noexcept
+      {
+	_M_rep |= __r._M_rep;
+	return *this;
+      }
+
+      constexpr __max_diff_type&
+      operator^=(const __max_diff_type& __r) noexcept
+      {
+	_M_rep ^= __r._M_rep;
+	return *this;
+      }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator+=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a + __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator-=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a - __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator*=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a * __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator/=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a / __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator%=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a % __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator&=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a & __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator|=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a | __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator^=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a ^ __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator<<=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a << __b)); }
+
+      template<typename _Tp> requires integral<_Tp> || __is_int128<_Tp>
+	friend constexpr _Tp&
+	operator>>=(_Tp& __a, const __max_diff_type& __b) noexcept
+	{ return (__a = static_cast<_Tp>(__a >> __b)); }
+
+      friend constexpr __max_diff_type
+      operator+(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l += __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator-(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l -= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator*(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l *= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator/(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l /= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator%(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l %= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator<<(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l <<= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator>>(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l >>= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator&(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l &= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator|(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l |= __r;
+	return __l;
+      }
+
+      friend constexpr __max_diff_type
+      operator^(__max_diff_type __l, const __max_diff_type& __r) noexcept
+      {
+	__l ^= __r;
+	return __l;
+      }
+
+      friend constexpr bool
+      operator==(const __max_diff_type& __l, const __max_diff_type& __r) noexcept
+      { return __l._M_rep == __r._M_rep; }
+
+      friend constexpr bool
+      operator!=(const __max_diff_type& __l, const __max_diff_type& __r) noexcept
+      { return !(__l == __r); }
+
+      constexpr bool
+      operator<(const __max_diff_type& __r) const noexcept
+      {
+	const auto __lsign = _M_rep._M_msb;
+	const auto __rsign = __r._M_rep._M_msb;
+	if (__lsign ^ __rsign)
+	  return __lsign;
+	else
+	  return _M_rep < __r._M_rep;
+      }
+
+      friend constexpr bool
+      operator>(const __max_diff_type& __l, const __max_diff_type& __r) noexcept
+      { return __r < __l; }
+
+      friend constexpr bool
+      operator<=(const __max_diff_type& __l, const __max_diff_type& __r) noexcept
+      { return !(__r < __l); }
+
+      friend constexpr bool
+      operator>=(const __max_diff_type& __l, const __max_diff_type& __r) noexcept
+      { return !(__l < __r); }
+
+    private:
+      __max_size_type _M_rep = 0;
+
+      friend class __max_size_type;
+    };
+
+    constexpr
+    __max_size_type::__max_size_type(const __max_diff_type& __d) noexcept
+      : __max_size_type(__d._M_rep)
+    { }
+
+  } // namespace __detail
+} // namespace ranges
+
+  template<>
+    struct numeric_limits<ranges::__detail::__max_size_type>
+    {
+      using _Sp = ranges::__detail::__max_size_type;
+      static constexpr bool is_specialized = true;
+      static constexpr bool is_signed = false;
+      static constexpr bool is_integer = true;
+      static constexpr bool is_exact = true;
+#if __SIZEOF_INT128__
+      static_assert(same_as<_Sp::__rep, unsigned __int128>);
+      static constexpr int digits = 129;
+#else
+      static_assert(same_as<_Sp::__rep, unsigned long long>);
+      static constexpr int digits
+	= __gnu_cxx::__int_traits<unsigned long long>::__digits + 1;
+#endif
+      static constexpr int digits10
+	= static_cast<int>(digits * numbers::ln2 / numbers::ln10);
+
+      static constexpr _Sp
+      min() noexcept
+      { return 0; }
+
+      static constexpr _Sp
+      max() noexcept
+      { return _Sp(static_cast<_Sp::__rep>(-1), 1); }
+
+      static constexpr _Sp
+      lowest() noexcept
+      { return min(); }
+    };
+
+  template<>
+    struct numeric_limits<ranges::__detail::__max_diff_type>
+    {
+      using _Dp = ranges::__detail::__max_diff_type;
+      using _Sp = ranges::__detail::__max_size_type;
+      static constexpr bool is_specialized = true;
+      static constexpr bool is_signed = true;
+      static constexpr bool is_integer = true;
+      static constexpr bool is_exact = true;
+      static constexpr int digits = numeric_limits<_Sp>::digits - 1;
+      static constexpr int digits10
+	= static_cast<int>(digits * numbers::ln2 / numbers::ln10);
+
+      static constexpr _Dp
+      min() noexcept
+      { return _Dp(_Sp(0, 1)); }
+
+      static constexpr _Dp
+      max() noexcept
+      { return _Dp(_Sp(static_cast<_Sp::__rep>(-1), 0)); }
+
+      static constexpr _Dp
+      lowest() noexcept
+      { return min(); }
+    };
+
+_GLIBCXX_END_NAMESPACE_VERSION
+} // namespace
+
+#endif // C++20 && library concepts
+#endif // _GLIBCXX_MAX_SIZE_TYPE_H
diff --git a/libstdc++-v3/include/bits/range_access.h b/libstdc++-v3/include/bits/range_access.h
index 3919183..3eb1f2f 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -36,6 +36,9 @@
 #include <initializer_list>
 #include <bits/iterator_concepts.h>
 #include <ext/numeric_traits.h>
+#if __cplusplus > 201703L
+#include <bits/max_size_type.h>
+#endif
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
@@ -352,6 +355,14 @@ namespace ranges
 
   namespace __detail
   {
+    constexpr __max_size_type
+    __to_unsigned_like(__max_size_type __t) noexcept
+    { return __t; }
+
+    constexpr __max_size_type
+    __to_unsigned_like(__max_diff_type __t) noexcept
+    { return __max_size_type(__t); }
+
     template<integral _Tp>
       constexpr make_unsigned_t<_Tp>
       __to_unsigned_like(_Tp __t) noexcept
diff --git a/libstdc++-v3/testsuite/std/ranges/iota/difference_type.cc b/libstdc++-v3/testsuite/std/ranges/iota/difference_type.cc
new file mode 100644
index 0000000..6eb1a16
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/iota/difference_type.cc
@@ -0,0 +1,57 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <ranges>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  using I = unsigned long long;
+  auto imax = std::numeric_limits<I>::max();
+  std::ranges::iota_view<I, I> i(0, imax);
+  auto begin = i.begin();
+  static_assert( std::input_or_output_iterator<decltype(begin)> );
+  auto size = std::ranges::end(i) - std::ranges::begin(i);
+  VERIFY( size > 0 );
+  VERIFY( size == imax );
+}
+
+void
+test02()
+{
+#if !defined(__STRICT_ANSI__) && __SIZEOF_INT128__
+  using I = unsigned __int128;
+  auto imax = std::numeric_limits<I>::max();
+  std::ranges::iota_view<I, I> i(0, imax);
+  auto begin = i.begin();
+  static_assert( std::input_or_output_iterator<decltype(begin)> );
+  auto size = std::ranges::end(i) - std::ranges::begin(i);
+  VERIFY( size > 0 );
+  VERIFY( size == imax );
+#endif
+}
+
+int
+main()
+{
+  test01();
+  test02();
+}
diff --git a/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
new file mode 100644
index 0000000..cb94440
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/iota/max_size_type.cc
@@ -0,0 +1,376 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <limits>
+#include <ranges>
+#include <testsuite_hooks.h>
+
+using max_size_t = std::ranges::__detail::__max_size_type;
+using max_diff_t = std::ranges::__detail::__max_diff_type;
+using rep_t = max_size_t::__rep;
+
+static_assert(sizeof(max_size_t) == sizeof(max_diff_t));
+
+static_assert(std::regular<max_size_t>);
+static_assert(std::totally_ordered<max_size_t>);
+
+static_assert(std::regular<max_diff_t>);
+static_assert(std::totally_ordered<max_diff_t>);
+
+// We can't use numeric_limits<rep_t>::max() here because __int128 is an
+// integral type only in GNU mode.
+constexpr max_size_t mu = max_size_t(~rep_t(0));
+constexpr max_size_t ou = 1;
+constexpr max_diff_t ns = -1;
+
+void
+test01()
+{
+  static_assert(max_size_t(7) % 3 == 1);
+  static_assert(max_size_t(7) % 4 == 3);
+
+  static_assert(-max_diff_t(1) == max_diff_t(-1));
+  static_assert(max_diff_t(3) % 2 == 1);
+  static_assert(max_diff_t(-3) / 2 == -1);
+  static_assert(max_diff_t(-3) % 2 == -1);
+  static_assert(max_diff_t(3) % -2 == 1);
+  static_assert(max_diff_t(-3) << 1 == -6);
+  static_assert(max_diff_t(-3) >> 1 == -2);
+  static_assert(max_diff_t(3) >> 1 == 1);
+  static_assert(max_diff_t(3) >> 2 == 0);
+
+  static_assert(max_diff_t(-5) / 3 == -1);
+  static_assert(max_diff_t(5) / -3 == -1);
+  static_assert(max_diff_t(-5) / -3 == 1);
+  static_assert(max_diff_t(5) / 3 == 1);
+
+  static_assert(max_diff_t(-6) / 3 == -2);
+  static_assert(max_diff_t(6) / -3 == -2);
+  static_assert(max_diff_t(-6) / -3 == 2);
+  static_assert(max_diff_t(6) / 3 == 2);
+
+  static_assert(~max_size_t(-3) == 2);
+  static_assert(~max_diff_t(-3) == 2);
+
+  static_assert(max_diff_t(1) < max_diff_t(3));
+  static_assert(max_diff_t(-1) < max_diff_t(3));
+  static_assert(max_diff_t(1) > max_diff_t(-3));
+  static_assert(max_diff_t(-1) > max_diff_t(-3));
+
+  static_assert(max_diff_t(mu)/-1 == -max_diff_t(mu));
+  static_assert(-max_diff_t(mu)/1 == -max_diff_t(mu));
+  static_assert(max_diff_t(mu)>>1 == max_diff_t(mu)/2);
+  static_assert(-max_diff_t(mu+1) == max_diff_t(mu+1));
+  static_assert(-(mu+1) == mu+1);
+  static_assert((mu+1)<<1 == 0);
+  static_assert(max_diff_t(mu+1)<<1 == 0);
+  static_assert(max_diff_t(mu+1)>>1 < 0);
+
+  static_assert(int(max_diff_t(mu+1)) == 0);
+  static_assert(rep_t(max_diff_t(mu+1)) == 0);
+  static_assert(int(max_diff_t(mu)) == -1);
+  static_assert(rep_t(max_diff_t(mu)) == rep_t(-1));
+
+  static_assert(2*mu+1 > 2*mu);
+  static_assert(~(2*mu+1) == 0);
+  static_assert(mu/mu == 1);
+  static_assert(2*mu > mu);
+  static_assert(2*mu-mu == mu);
+  static_assert((2*mu)/mu == 2);
+  static_assert((2*mu+1)/mu == 2);
+  static_assert((2*mu-1)/(mu-1) == 2);
+  static_assert((2*mu-1)/mu == 1);
+  static_assert((2*mu+-1)/mu == 1);
+  static_assert(2*mu-1 < 2*mu);
+  static_assert(2*mu-1 <= 2*mu);
+  static_assert(2*mu+1 > 2*mu);
+  static_assert(2*mu+1 >= 2*mu);
+  static_assert((2*mu)/1 == 2*mu);
+  static_assert(mu/mu-1 == 0);
+  static_assert(mu*0 == 0);
+  static_assert((2*mu-1)*0 == 0);
+  static_assert((2*mu-1)>>1 == mu-1);
+  static_assert(mu+-1+1 == mu);
+  static_assert(mu+1+-1 == mu);
+  static_assert(mu+1);
+  static_assert((2*mu)/2 == mu);
+  static_assert((2*mu)>>1 == mu);
+  static_assert((mu<<1)>>1 == mu);
+  static_assert(1/mu == 0);
+  static_assert(mu/1 == mu);
+  static_assert(((mu+1)|mu) == -1);
+  static_assert((mu+1)+(mu+1) < mu+1);
+
+  static_assert(max_size_t(ns) == -1);
+  static_assert(-max_diff_t(ou) == -1);
+  static_assert(-max_diff_t(-ou) == 1);
+  static_assert(max_size_t(-max_diff_t(-ou)) == 1);
+  static_assert(ns*ns == max_diff_t(ou));
+  static_assert(max_size_t(ns)*max_size_t(ns) == ou);
+  static_assert(-max_diff_t(0) == max_diff_t(0));
+  static_assert(-ou-ou == -2*ou);
+
+  static_assert(int(ns) == -1);
+  static_assert(rep_t(ns) == rep_t(-1));
+
+  static_assert(max_size_t() == 0);
+  static_assert(max_diff_t() == 0);
+
+  auto f = [] (auto a) { a /= a; return a; };
+  static_assert(f(max_size_t(5)) == 1);
+  static_assert(f(max_size_t(-5)) == 1);
+  static_assert(f(max_diff_t(5)) == 1);
+
+  auto g = [] (auto a) { a >>= a; return a; };
+  static_assert(g(max_size_t(5)) == 0);
+  static_assert(g(max_diff_t(5)) == 0);
+
+  auto h = [] (auto a) { a <<= a; return a; };
+  static_assert(h(max_size_t(3)) == 24);
+  static_assert(h(max_diff_t(3)) == 24);
+}
+
+template<bool signed_p, bool shorten_p>
+void
+test02()
+{
+  using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>;
+  using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>;
+  using shorten_type = std::conditional_t<shorten_p, hw_type, max_type>;
+  const int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__;
+  const int limit = 1000;
+  const int log2_limit = 10;
+  static_assert((1 << log2_limit) >= limit);
+  const int min = (signed_p ? -limit : 0);
+  const int max = limit;
+  for (hw_type i = min; i <= max; i++)
+    {
+      bool ok = true;
+      if (signed_p || shorten_p)
+	{
+	  ok &= (~i == shorten_type(~max_type(i)));
+	  ok &= (-i == shorten_type(-max_type(i)));
+	}
+      for (hw_type j = min; j <= max; j++)
+	{
+	  ok &= (i*j == shorten_type(max_type(i)*j));
+	  ok &= (i+j == shorten_type(max_type(i)+j));
+	  if (j != 0)
+	    {
+	      ok &= (i/j == shorten_type(max_type(i)/j));
+	      ok &= (i%j == shorten_type(max_type(i)%j));
+	    }
+	  if (signed_p || shorten_p)
+	    ok &= (i-j == shorten_type(max_type(i)-j));
+	  ok &= ((i&j) == shorten_type(max_type(i)&j));
+	  ok &= ((i|j) == shorten_type(max_type(i)|j));
+	  ok &= ((i^j) == shorten_type(max_type(i)^j));
+	  if (j >= 0 && j < hw_type(hw_type_bit_size)
+	      && (shorten_p || j < hw_type(hw_type_bit_size) - log2_limit))
+	    {
+	      ok &= ((i>>j) == shorten_type(max_type(i)>>j));
+	      ok &= ((i<<j) == shorten_type(max_type(i)<<j));
+	    }
+	  ok &= (i>j) == (max_type(i) > j);
+	  ok &= (i<j) == (max_type(i) < j);
+	  ok &= (i>=j) == (max_type(i) >= j);
+	  ok &= (i<=j) == (max_type(i) <= j);
+	  ok &= (i==j) == (max_type(i) == j);
+	  ok &= (i!=j) == (max_type(i) != j);
+	  if (!ok)
+	    {
+	      fprintf(stderr,
+		      "Inconsistency found: %d %d %lld %lld\n",
+		      signed_p, shorten_p, (long long)i, (long long)j) ;
+	       VERIFY(0);
+	    }
+	}
+    }
+}
+
+template<bool signed_p, bool toggle_base_p>
+void
+test03()
+{
+  using hw_type = std::conditional_t<signed_p, signed rep_t, rep_t>;
+  using max_type = std::conditional_t<signed_p, max_diff_t, max_size_t>;
+  using base_type = std::conditional_t<toggle_base_p, hw_type, max_type>;
+  constexpr int hw_type_bit_size = sizeof(hw_type) * __CHAR_BIT__;
+  constexpr int limit = 1000;
+  constexpr int log2_limit = 10;
+  static_assert((1 << log2_limit) >= limit);
+  const int min = (signed_p ? -limit : 0);
+  const int max = limit;
+  for (hw_type i = min; i <= max; i++)
+    {
+      bool ok = true;
+      base_type k;
+      for (hw_type j = min; j <= max; j++)
+	{
+	  k = i; k *= j;
+	  ok &= (k == (max_type(i)*j));
+	  k = i; k += j;
+	  ok &= (k == (max_type(i)+j));
+	  if (j != 0)
+	    {
+	      k = i; k /= j;
+	      ok &= (k == (max_type(i)/j));
+	      k = i; k %= j;
+	      ok &= (k == (max_type(i)%j));
+	    }
+	  if (signed_p)
+	    {
+	      k = i; k -= j;
+	      ok &= (k == (max_type(i)-j));
+	    }
+	  k = i; k &= j;
+	  ok &= (k == (max_type(i)&j));
+	  k = i; k |= j;
+	  ok &= (k == (max_type(i)|j));
+	  k = i; k ^= j;
+	  ok &= (k == (max_type(i)^j));
+	  if (j >= 0 && j < hw_type(hw_type_bit_size)
+	      && (!toggle_base_p || j < hw_type(hw_type_bit_size) - log2_limit))
+	    {
+	      k = i; k >>= j;
+	      ok &= (k == (max_type(i)>>j));
+	      k = i; k <<= j;
+	      ok &= (k == (max_type(i)<<j));
+	    }
+	  if (!ok)
+	    {
+	      fprintf(stderr,
+		      "Inconsistency found: %d %d %lld %lld\n",
+		      signed_p, toggle_base_p, (long long)i, (long long)j) ;
+	       VERIFY(0);
+	    }
+	}
+    }
+}
+
+void
+test04()
+{
+  constexpr int limit = 1000;
+  for (int i = -limit; i <= limit; i++)
+    {
+      VERIFY( -max_size_t(-i) == i );
+      for (int j = i; j <= limit; j++)
+	{
+	  VERIFY( max_size_t(-i) * max_size_t(-j) == i*j );
+	  VERIFY( max_size_t(-j) * max_size_t(-i) == j*i );
+	  VERIFY( rep_t(((mu+1)+i)*((mu+1)+j)) == rep_t(i*j) );
+	  VERIFY( rep_t(((mu+1)+j)*((mu+1)+i)) == rep_t(j*i) );
+	  if (i >= 0 && j > 0)
+	    {
+	      auto r = (mu+i)-((mu+i)/j)*j;
+	      VERIFY( r >= 0 && r < j );
+	      VERIFY( r == (mu+i)%j );
+	    }
+	}
+    }
+}
+
+void
+test05()
+{
+#if __SIZEOF_INT128__
+  max_size_t x = 0;
+  x = static_cast<__int128>(0);
+  x = static_cast<unsigned __int128>(0);
+
+  max_diff_t y = 0;
+  y = static_cast<__int128>(0);;
+  y = static_cast<unsigned __int128>(0);
+#endif
+}
+
+using std::numeric_limits;
+
+static_assert(numeric_limits<max_size_t>::is_specialized);
+static_assert(!numeric_limits<max_size_t>::is_signed);
+static_assert(numeric_limits<max_size_t>::is_integer);
+static_assert(numeric_limits<max_size_t>::is_exact);
+// We can't unconditionally use numeric_limits here because __int128 is an
+// integral type only in GNU mode.
+#if __SIZEOF_INT128__
+static_assert(numeric_limits<max_size_t>::digits == 129);
+static_assert(numeric_limits<max_size_t>::digits10 == 38);
+static_assert(numeric_limits<max_size_t>::max()
+	      == 2*max_size_t(~rep_t(0)) + 1);
+#else
+static_assert(numeric_limits<max_size_t>::digits
+	      == numeric_limits<rep_t>::digits + 1);
+static_assert(numeric_limits<max_size_t>::digits10
+	      == numeric_limits<rep_t>::digits10);
+static_assert(numeric_limits<max_size_t>::max()
+	      == 2*max_size_t(numeric_limits<rep_t>::max())+1);
+#endif
+static_assert(numeric_limits<max_size_t>::min() == 0);
+static_assert(numeric_limits<max_size_t>::max()
+	      == max_size_t(-1));
+static_assert((numeric_limits<max_size_t>::max()
+	       >> (numeric_limits<max_size_t>::digits-1)) == 1);
+static_assert(numeric_limits<max_size_t>::lowest()
+	      == numeric_limits<max_size_t>::min());
+
+static_assert(numeric_limits<max_diff_t>::is_specialized);
+static_assert(numeric_limits<max_diff_t>::is_signed);
+static_assert(numeric_limits<max_diff_t>::is_integer);
+static_assert(numeric_limits<max_diff_t>::is_exact);
+static_assert(numeric_limits<max_diff_t>::digits
+	      == numeric_limits<max_size_t>::digits - 1);
+static_assert(numeric_limits<max_diff_t>::digits10
+	      == numeric_limits<max_size_t>::digits10);
+// We can't unconditionally use numeric_limits here because __int128 is an
+// integral type only in GNU mode.
+#if __SIZEOF_INT128__
+static_assert(numeric_limits<max_diff_t>::min() == -max_diff_t(~rep_t(0))-1);
+static_assert(numeric_limits<max_diff_t>::max() == ~rep_t(0));
+#else
+static_assert(numeric_limits<max_diff_t>::min()
+	      == -max_diff_t(numeric_limits<rep_t>::max())-1);
+static_assert(numeric_limits<max_diff_t>::max()
+	      == numeric_limits<rep_t>::max());
+#endif
+static_assert(numeric_limits<max_diff_t>::lowest()
+	      == numeric_limits<max_diff_t>::min());
+static_assert(max_diff_t(max_size_t(1)
+			 << (numeric_limits<max_size_t>::digits-1))
+	      == numeric_limits<max_diff_t>::min());
+
+int
+main()
+{
+  test01();
+
+  test02<false,false>();
+  test02<false,true>();
+  test02<true,false>();
+  test02<true,true>();
+
+  test03<false,false>();
+  test03<false,true>();
+  test03<true,false>();
+  test03<true,true>();
+
+  test04();
+  test05();
+}