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

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.

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 (back_insert_iterator): Remove the
	default ctor.
	(front_insert_iterator): Likewise.
	(insert_iterator): Likewise.  Remove NSDMIs.
	(common_iterator): Constrain the default ctor.
	(counted_iterator): Likewise.
	* include/bits/stream_iterator.h (ostream_iterator): Remove the
	default ctor.
	* 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.
	(basic_istream_view): Remove the default ctor.  Remove NSDMIs.
	Remove redundant checks for empty _M_stream.
	(basic_istream_view::_Iterator): Likewise.
	(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/24_iterators/back_insert_iterator/constexpr.cc:
	Don't attempt to default construct a back_insert_iterator.
	* testsuite/24_iterators/front_insert_iterator/constexpr.cc:
	Don't attempt to default construct a front_insert_iterator.
	* testsuite/24_iterators/insert_iterator/constexpr.cc:
	Don't attempt to default construct an insert_iterator.
	* testsuite/24_iterators/ostream_iterator/requirements/constexpr.cc:
	Remove this test for default constructibility of ostream_iterator.
	* testsuite/std/ranges/97600.cc: Don't attempt to default
	construct a basic_istream_view.
	* 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.

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

@@ -241,7 +241,7 @@ 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

@@ -639,8 +639,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef _Container          container_type;
 #if __cplusplus > 201703L
       using difference_type = ptrdiff_t;
-
-      constexpr back_insert_iterator() noexcept : container(nullptr) { }
 #endif
 
       /// The only way to create this %iterator is with a container.
@@ -742,8 +740,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       typedef _Container          container_type;
 #if __cplusplus > 201703L
       using difference_type = ptrdiff_t;
-
-      constexpr front_insert_iterator() noexcept : container(nullptr) { }
 #endif
 
       /// The only way to create this %iterator is with a container.
@@ -843,26 +839,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
 #if __cplusplus > 201703L && defined __cpp_lib_concepts
       using _Iter = std::__detail::__range_iter_t<_Container>;
-
-    protected:
-      _Container* container = nullptr;
-      _Iter iter = _Iter();
 #else
       typedef typename _Container::iterator		_Iter;
-
+#endif
     protected:
       _Container* container;
       _Iter iter;
-#endif
 
     public:
       /// A nested typedef for the type of whatever container you used.
       typedef _Container          container_type;
 
 #if __cplusplus > 201703L && defined __cpp_lib_concepts
       using difference_type = ptrdiff_t;
-
-      insert_iterator() = default;
 #endif
 
       /**
@@ -1740,6 +1729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr
     common_iterator()
     noexcept(is_nothrow_default_constructible_v<_It>)
+    requires default_initializable<_It>
     : _M_it(), _M_index(0)
     { }
 
@@ -2117,7 +2107,7 @@ _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/bits/stream_iterator.h

@@ -192,11 +192,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       const _CharT*	_M_string;
 
     public:
-#if __cplusplus > 201703L
-      constexpr ostream_iterator() noexcept
-      : _M_stream(nullptr), _M_string(nullptr) { }
-#endif
-
       /// Construct from an ostream.
       ostream_iterator(ostream_type& __s)
       : _M_stream(std::__addressof(__s)), _M_string(0) {}