libstdc++: Implement P2325 changes to default-constructibility of views

NB: This backport of r12-1606 to the 11 branch deliberately omits parts
of P2325R3 so as to maximize backward compatibility with pre-P2325R3 code.
In particular, we don't remove the default ctors for back_insert_iterator,
front_insert_iterator, ostream_iterator, ref_view and basic_istream_view.

===

This implements the wording changes of P2325R3 "Views should not be
required to be default constructible".  Changes are relatively
straightforward, besides perhaps those to __box (which now stands
for copyable-box instead of semiregular-box) and __non_propagating_cache.

For __box, this patch implements the recommended practice to also avoid
std::optional when the boxed type is nothrow_move/copy_constructible.

For __non_propagating_cache, now that it's used by split_view::_M_current,
we need to add assignment from a value of the underlying type to the
subset of the std::optional API implemented for the cache (needed by
split_view::begin()).  Hence the new __non_propagating_cache::operator=
overload.

In passing, this fixes the undesirable list-init in the constructors of
the partial specialization of __box as reported in PR100475 comment gcc-mirror#7.

	PR libstdc++/103904

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (weakly_incrementable): Remove
	default_initializable requirement.
	* include/bits/ranges_base.h (ranges::view): Likewise.
	* include/bits/ranges_util.h (subrange): Constrain the default
	ctor.
	* include/bits/stl_iterator.h (common_iterator): Constrain the
	default ctor.
	(counted_iterator): Likewise.
	* include/std/ranges (__detail::__box::operator=): Handle
	self-assignment in the primary template.
	(__detail::__box): In the partial specialization: adjust
	constraints as per P2325.  Add specialized operator= for the
	case when the wrapped type is not copyable.  Constrain the
	default ctor.  Avoid list-initialization.
	(single_view): Constraint the default ctor.
	(iota_view): Relax semiregular constraint to copyable.
	Constrain the default ctor.
	(iota_view::_Iterator): Constraint the default ctor.
	(ref_view): Remove the default ctor.  Remove NSDMIs.
	(ref_view::_Iterator): Constrain the default ctor.
	(__detail::__non_propagating_cache::operator=): Define overload
	for assigning from a value of the underlying type.
	(filter_view): Likewise.
	(filter_view::_Iterator): Likewise.
	(transform_view): Likewise.
	(transform_view::_Iterator): Likewise.
	(take_view): Likewise.
	(take_view::_Iterator): Likewise.
	(take_while_view): Likewise.
	(take_while_view::_Iterator): Likewise.
	(drop_while_view): Likewise.
	(drop_while_view::_Iterator): Likewise.
	(join_view): Likewise.
	(split_view::_OuterIter::__current): Adjust after changing the
	type of _M_current.
	(split_view::_M_current): Wrap it in a __non_propagating_cache.
	(split_view::split_view): Constrain the default ctor.
	(common_view): Constrain the default ctor.
	(reverse_view): Likewise.
	(elements_view): Likewise.
	* include/std/span (enable_view<span<_ElementType, _Extent>>):
	Define this partial specialization to true unconditionally.
	* include/std/version (__cpp_lib_ranges): Adjust value.
	* testsuite/std/ranges/adaptors/detail/semiregular_box.cc:
	Rename to ...
	* testsuite/std/ranges/adaptors/detail/copyable_box.cc: ... this.
	(test02): Adjust now that __box is copyable-box not
	semiregular-box.
	(test03): New test.
	* testsuite/std/ranges/p2325.cc: New test.
	* testsuite/std/ranges/single_view.cc (test06): New test.
	* testsuite/std/ranges/view.cc: Adjust now that view doesn't
	require default_initializable.

(cherry picked from commit 4b4f566)

Author: Patrick Palka

libstdc++-v3/include/bits/iterator_concepts.h

@@ -594,8 +594,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   /// Requirements on types that can be incremented with ++.
   template<typename _Iter>
-    concept weakly_incrementable = default_initializable<_Iter>
-      && movable<_Iter>
+    concept weakly_incrementable = movable<_Iter>
       && requires(_Iter __i)
       {
 	typename iter_difference_t<_Iter>;

libstdc++-v3/include/bits/ranges_base.h

@@ -619,8 +619,7 @@ namespace ranges
   /// [range.view] The ranges::view concept.
   template<typename _Tp>
     concept view
-      = range<_Tp> && movable<_Tp> && default_initializable<_Tp>
-	&& enable_view<_Tp>;
+      = range<_Tp> && movable<_Tp> && enable_view<_Tp>;
 
   // [range.refinements]
 

libstdc++-v3/include/bits/ranges_util.h

@@ -219,7 +219,7 @@ namespace ranges
       [[no_unique_address]] _Size<__size_type> _M_size = {};
 
     public:
-      subrange() = default;
+      subrange() requires default_initializable<_It> = default;
 
       constexpr
       subrange(__detail::__convertible_to_non_slicing<_It> auto __i, _Sent __s)

libstdc++-v3/include/bits/stl_iterator.h

@@ -1741,6 +1741,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr
     common_iterator()
     noexcept(is_nothrow_default_constructible_v<_It>)
+    requires default_initializable<_It>
     : _M_it(), _M_index(0)
     { }
 
@@ -2118,7 +2119,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // iterator_concept defined in __counted_iter_concept
       // iterator_category defined in __counted_iter_cat
 
-      constexpr counted_iterator() = default;
+      constexpr counted_iterator() requires default_initializable<_It> = default;
 
       constexpr
       counted_iterator(_It __i, iter_difference_t<_It> __n)

libstdc++-v3/include/std/ranges

@@ -113,10 +113,13 @@ namespace ranges
 	noexcept(is_nothrow_copy_constructible_v<_Tp>)
 	requires (!copyable<_Tp>)
 	{
-	  if ((bool)__that)
-	    this->emplace(*__that);
-	  else
-	    this->reset();
+	  if (this != std::__addressof(__that))
+	    {
+	      if ((bool)__that)
+		this->emplace(*__that);
+	      else
+		this->reset();
+	    }
 	  return *this;
 	}
 
@@ -125,37 +128,42 @@ namespace ranges
 	noexcept(is_nothrow_move_constructible_v<_Tp>)
 	requires (!movable<_Tp>)
 	{
-	  if ((bool)__that)
-	    this->emplace(std::move(*__that));
-	  else
-	    this->reset();
+	  if (this != std::__addressof(__that))
+	    {
+	      if ((bool)__that)
+		this->emplace(std::move(*__that));
+	      else
+		this->reset();
+	    }
 	  return *this;
 	}
       };
 
-    // For types which are already semiregular, this specialization of the
-    // semiregular wrapper stores the object directly without going through
+    // For types which are already copyable, this specialization of the
+    // copyable wrapper stores the object directly without going through
     // std::optional.  It provides just the subset of the primary template's
     // API that we currently use.
-    template<__boxable _Tp> requires semiregular<_Tp>
+    template<__boxable _Tp>
+      requires copyable<_Tp> || (is_nothrow_move_constructible_v<_Tp>
+				 && is_nothrow_copy_constructible_v<_Tp>)
       struct __box<_Tp>
       {
       private:
 	[[no_unique_address]] _Tp _M_value = _Tp();
 
       public:
-	__box() = default;
+	__box() requires default_initializable<_Tp> = default;
 
 	constexpr explicit
 	__box(const _Tp& __t)
 	noexcept(is_nothrow_copy_constructible_v<_Tp>)
-	: _M_value{__t}
+	: _M_value(__t)
 	{ }
 
 	constexpr explicit
 	__box(_Tp&& __t)
 	noexcept(is_nothrow_move_constructible_v<_Tp>)
-	: _M_value{std::move(__t)}
+	: _M_value(std::move(__t))
 	{ }
 
 	template<typename... _Args>
@@ -166,6 +174,38 @@ namespace ranges
 	  : _M_value(std::forward<_Args>(__args)...)
 	  { }
 
+	__box(const __box&) = default;
+	__box(__box&&) = default;
+	__box& operator=(const __box&) requires copyable<_Tp> = default;
+	__box& operator=(__box&&) requires copyable<_Tp> = default;
+
+	// When _Tp is nothrow_copy_constructible but not copy_assignable,
+	// copy assignment is implemented via destroy-then-copy-construct.
+	constexpr __box&
+	operator=(const __box& __that) noexcept
+	{
+	  static_assert(is_nothrow_copy_constructible_v<_Tp>);
+	  if (this != std::__addressof(__that))
+	    {
+	      _M_value.~_Tp();
+	      std::construct_at(std::__addressof(_M_value), *__that);
+	    }
+	  return *this;
+	}
+
+	// Likewise for move assignment.
+	constexpr __box&
+	operator=(__box&& __that) noexcept
+	{
+	  static_assert(is_nothrow_move_constructible_v<_Tp>);
+	  if (this != std::__addressof(__that))
+	    {
+	      _M_value.~_Tp();
+	      std::construct_at(std::__addressof(_M_value), std::move(*__that));
+	    }
+	  return *this;
+	}
+
 	constexpr bool
 	has_value() const noexcept
 	{ return true; };
@@ -193,7 +233,7 @@ namespace ranges
     class single_view : public view_interface<single_view<_Tp>>
     {
     public:
-      single_view() = default;
+      single_view() requires default_initializable<_Tp> = default;
 
       constexpr explicit
       single_view(const _Tp& __t)
@@ -305,7 +345,7 @@ namespace ranges
   template<weakly_incrementable _Winc,
 	   semiregular _Bound = unreachable_sentinel_t>
     requires std::__detail::__weakly_eq_cmp_with<_Winc, _Bound>
-      && semiregular<_Winc>
+      && copyable<_Winc>
     class iota_view : public view_interface<iota_view<_Winc, _Bound>>
     {
     private:
@@ -334,7 +374,7 @@ namespace ranges
 	using value_type = _Winc;
 	using difference_type = __detail::__iota_diff_t<_Winc>;
 
-	_Iterator() = default;
+	_Iterator() requires default_initializable<_Winc> = default;
 
 	constexpr explicit
 	_Iterator(_Winc __value)
@@ -531,7 +571,7 @@ namespace ranges
       [[no_unique_address]] _Bound _M_bound = _Bound();
 
     public:
-      iota_view() = default;
+      iota_view() requires default_initializable<_Winc> = default;
 
       constexpr explicit
       iota_view(_Winc __value)
@@ -1140,6 +1180,16 @@ namespace views::__adaptor
 	  return *this;
 	}
 
+	constexpr __non_propagating_cache&
+	operator=(_Tp __val)
+	{
+	  this->_M_reset();
+	  std::construct_at(std::__addressof(this->_M_payload._M_payload),
+			    std::in_place, std::move(__val));
+	  this->_M_payload._M_engaged = true;
+	  return *this;
+	}
+
 	constexpr _Tp&
 	operator*() noexcept
 	{ return this->_M_get(); }
@@ -1317,7 +1367,7 @@ namespace views::__adaptor
 	using value_type = range_value_t<_Vp>;
 	using difference_type = range_difference_t<_Vp>;
 
-	_Iterator() = default;
+	_Iterator() requires default_initializable<_Vp_iter> = default;
 
 	constexpr
 	_Iterator(filter_view* __parent, _Vp_iter __current)
@@ -1429,7 +1479,9 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      filter_view() = default;
+      filter_view() requires (default_initializable<_Vp>
+			      && default_initializable<_Pred>)
+	= default;
 
       constexpr
       filter_view(_Vp __base, _Pred __pred)
@@ -1578,7 +1630,7 @@ namespace views::__adaptor
 	    = remove_cvref_t<invoke_result_t<_Fp&, range_reference_t<_Base>>>;
 	  using difference_type = range_difference_t<_Base>;
 
-	  _Iterator() = default;
+	  _Iterator() requires default_initializable<_Base_iter> = default;
 
 	  constexpr
 	  _Iterator(_Parent* __parent, _Base_iter __current)
@@ -1793,7 +1845,9 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      transform_view() = default;
+      transform_view() requires (default_initializable<_Vp>
+				 && default_initializable<_Fp>)
+	= default;
 
       constexpr
       transform_view(_Vp __base, _Fp __fun)
@@ -1928,7 +1982,7 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      take_view() = default;
+      take_view() requires default_initializable<_Vp> = default;
 
       constexpr
       take_view(_Vp base, range_difference_t<_Vp> __count)
@@ -2112,7 +2166,9 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      take_while_view() = default;
+      take_while_view() requires (default_initializable<_Vp>
+				  && default_initializable<_Pred>)
+	= default;
 
       constexpr
       take_while_view(_Vp base, _Pred __pred)
@@ -2200,7 +2256,7 @@ namespace views::__adaptor
 				      _M_cached_begin;
 
     public:
-      drop_view() = default;
+      drop_view() requires default_initializable<_Vp> = default;
 
       constexpr
       drop_view(_Vp __base, range_difference_t<_Vp> __count)
@@ -2316,7 +2372,9 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      drop_while_view() = default;
+      drop_while_view() requires (default_initializable<_Vp>
+				  && default_initializable<_Pred>)
+	= default;
 
       constexpr
       drop_while_view(_Vp __base, _Pred __pred)
@@ -2506,7 +2564,9 @@ namespace views::__adaptor
 	    = common_type_t<range_difference_t<_Base>,
 			    range_difference_t<range_reference_t<_Base>>>;
 
-	  _Iterator() = default;
+	  _Iterator() requires (default_initializable<_Outer_iter>
+				&& default_initializable<_Inner_iter>)
+	    = default;
 
 	  constexpr
 	  _Iterator(_Parent* __parent, _Outer_iter __outer)
@@ -2659,7 +2719,7 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      join_view() = default;
+      join_view() requires default_initializable<_Vp> = default;
 
       constexpr explicit
       join_view(_Vp __base)
@@ -2826,7 +2886,7 @@ namespace views::__adaptor
 	    if constexpr (forward_range<_Vp>)
 	      return _M_current;
 	    else
-	      return _M_parent->_M_current;
+	      return *_M_parent->_M_current;
 	  }
 
 	  constexpr auto&
@@ -2835,7 +2895,7 @@ namespace views::__adaptor
 	    if constexpr (forward_range<_Vp>)
 	      return _M_current;
 	    else
-	      return _M_parent->_M_current;
+	      return *_M_parent->_M_current;
 	  }
 
 	  _Parent* _M_parent = nullptr;
@@ -3083,12 +3143,14 @@ namespace views::__adaptor
       // XXX: _M_current is "present only if !forward_range<V>"
       [[no_unique_address]]
 	__detail::__maybe_present_t<!forward_range<_Vp>,
-				    iterator_t<_Vp>> _M_current;
+	  __detail::__non_propagating_cache<iterator_t<_Vp>>> _M_current;
       _Vp _M_base = _Vp();
 
 
     public:
-      split_view() = default;
+      split_view() requires (default_initializable<_Vp>
+			     && default_initializable<_Pattern>)
+	= default;
 
       constexpr
       split_view(_Vp __base, _Pattern __pattern)
@@ -3219,7 +3281,7 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      common_view() = default;
+      common_view() requires default_initializable<_Vp> = default;
 
       constexpr explicit
       common_view(_Vp __r)
@@ -3348,7 +3410,7 @@ namespace views::__adaptor
       _Vp _M_base = _Vp();
 
     public:
-      reverse_view() = default;
+      reverse_view() requires default_initializable<_Vp> = default;
 
       constexpr explicit
       reverse_view(_Vp __r)
@@ -3490,7 +3552,7 @@ namespace views::__adaptor
     class elements_view : public view_interface<elements_view<_Vp, _Nm>>
     {
     public:
-      elements_view() = default;
+      elements_view() requires default_initializable<_Vp> = default;
 
       constexpr explicit
       elements_view(_Vp base)
@@ -3611,7 +3673,7 @@ namespace views::__adaptor
 	    = remove_cvref_t<tuple_element_t<_Nm, range_value_t<_Base>>>;
 	  using difference_type = range_difference_t<_Base>;
 
-	  _Iterator() = default;
+	  _Iterator() requires default_initializable<iterator_t<_Base>> = default;
 
 	  constexpr explicit
 	  _Iterator(iterator_t<_Base> current)

libstdc++-v3/include/std/span

@@ -450,8 +450,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     // Opt-in to view concept
     template<typename _ElementType, size_t _Extent>
       inline constexpr bool
-	enable_view<span<_ElementType, _Extent>>
-	  = _Extent == 0 || _Extent == dynamic_extent;
+	enable_view<span<_ElementType, _Extent>> = true;
   }
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std

libstdc++-v3/include/std/version

@@ -247,7 +247,7 @@
 #define __cpp_lib_optional 202106L
 #define __cpp_lib_polymorphic_allocator 201902L
 #if __cpp_lib_concepts
-# define __cpp_lib_ranges 201911L
+# define __cpp_lib_ranges 202106L
 #endif
 #if __cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
 # define __cpp_lib_semaphore 201907L