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

NB: This backport of r12-1606 to the 10 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.
And in the 10 branch we we don't have __non_propagating_cache or the
partial specialization of __box, so the changes to them are omitted too.
Finally, we don't update __cpp_lib_ranges to 202106L because many
significant 202106 Ranges changes still aren't implemented in the 10
branch, e.g. P2281R1 and P2210R2.

===

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 #7.

	PR libstdc++/103904

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (weakly_incrementable): Remove
	default_initializable requirement.
	* include/bits/stl_iterator.h (common_iterator): Constrain the
	default ctor.
	(counted_iterator): Likewise.
	* include/std/ranges (ranges::view): Remove default_initializable
	requirement.
	(subrange): Constrain the default ctor.
	(__detail::__box::operator=): Handle self-assignment.
	(single_view): Constraint the default ctor.
	(iota_view): Relax semiregular constraint to copyable.
	Constrain the default ctor.
	(iota_view::_Iterator): Constraint the default ctor.
	(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): Constrain the default ctor.
	(common_view): Likewise.
	(reverse_view): Likewise.
	(elements_view): Likewise.
	(elements_view::_Iterator): Likewise.
	* include/std/span (enable_view<span<_ElementType, _Extent>>):
	Define this partial specialization to true unconditionally.
	* 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 4b4f5666b4c2f3aab2a9f3d53d394e390b9b682d)

diff --git a/libstdc++-v3/include/bits/iterator_concepts.h b/libstdc++-v3/include/bits/iterator_concepts.h
index 4f118df6b0274fd202c65ba86ed53d78e92b092f..a533ae2d279e39cd110b5173d387721c1bc8a526 100644 (file)
--- a/libstdc++-v3/include/bits/iterator_concepts.h
+++ b/libstdc++-v3/include/bits/iterator_concepts.h
@@ -573,8 +573,7 @@ _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>;

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index 91cfeccecf3857731c0a0388a6efb2d8f2984017..a46872fc3c2318707d47d2013149e1e7e525df07 100644 (file)
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -1729,6 +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)
     { }
 
@@ -2106,7 +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)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index cec775db3b1ac920422beb402915aebd93a2ad1d..4c08de39c1d677bf55e265126c8c48099f2f6206 100644 (file)
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -70,8 +70,7 @@ namespace ranges
 
   template<typename _Tp>
     concept view
-      = range<_Tp> && movable<_Tp> && default_initializable<_Tp>
-       && enable_view<_Tp>;
+      = range<_Tp> && movable<_Tp> && enable_view<_Tp>;
 
   /// A range which can be safely converted to a view.
   template<typename _Tp>
@@ -251,7 +250,7 @@ namespace ranges
       [[no_unique_address]] _Size<iter_difference_t<_It>> _M_size = {};
 
     public:
-      subrange() = default;
+      subrange() requires default_initializable<_It> = default;
 
       constexpr
       subrange(__detail::__convertible_to_non_slicing<_It> auto __i, _Sent __s)
@@ -486,10 +485,13 @@ 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;
        }
 
@@ -498,10 +500,13 @@ 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;
        }
       };
@@ -513,7 +518,7 @@ 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)
@@ -622,7 +627,7 @@ 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:
@@ -651,7 +656,7 @@ 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)
@@ -848,7 +853,7 @@ namespace ranges
       _Bound _M_bound = _Bound();
 
     public:
-      iota_view() = default;
+      iota_view() requires default_initializable<_Winc> = default;
 
       constexpr explicit
       iota_view(_Winc __value)
@@ -1475,7 +1480,7 @@ namespace views
        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)
@@ -1587,7 +1592,9 @@ namespace views
       [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
 
     public:
-      filter_view() = default;
+      filter_view() requires (default_initializable<_Vp>
+                             && default_initializable<_Pred>)
+       = default;
 
       constexpr
       filter_view(_Vp __base, _Pred __pred)
@@ -1718,7 +1725,7 @@ namespace views
            = 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)
@@ -1933,7 +1940,9 @@ namespace views
       __detail::__box<_Fp> _M_fun;
 
     public:
-      transform_view() = default;
+      transform_view() requires (default_initializable<_Vp>
+                                && default_initializable<_Fp>)
+       = default;
 
       constexpr
       transform_view(_Vp __base, _Fp __fun)
@@ -2050,7 +2059,7 @@ namespace views
       range_difference_t<_Vp> _M_count = 0;
 
     public:
-      take_view() = default;
+      take_view() requires default_initializable<_Vp> = default;
 
       constexpr
       take_view(_Vp base, range_difference_t<_Vp> __count)
@@ -2211,7 +2220,9 @@ namespace views
       __detail::__box<_Pred> _M_pred;
 
     public:
-      take_while_view() = default;
+      take_while_view() requires (default_initializable<_Vp>
+                                 && default_initializable<_Pred>)
+       = default;
 
       constexpr
       take_while_view(_Vp base, _Pred __pred)
@@ -2282,7 +2293,7 @@ namespace views
                                      _M_cached_begin;
 
     public:
-      drop_view() = default;
+      drop_view() requires default_initializable<_Vp> = default;
 
       constexpr
       drop_view(_Vp __base, range_difference_t<_Vp> __count)
@@ -2378,7 +2389,9 @@ namespace views
       [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
 
     public:
-      drop_while_view() = default;
+      drop_while_view() requires (default_initializable<_Vp>
+                                 && default_initializable<_Pred>)
+       = default;
 
       constexpr
       drop_while_view(_Vp __base, _Pred __pred)
@@ -2553,7 +2566,9 @@ namespace views
            = 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)
@@ -2709,7 +2724,7 @@ namespace views
                                    views::all_t<_InnerRange>> _M_inner;
 
     public:
-      join_view() = default;
+      join_view() requires default_initializable<_Vp> = default;
 
       constexpr explicit
       join_view(_Vp __base)
@@ -3123,7 +3138,10 @@ namespace views
 
 
     public:
-      split_view() = default;
+      split_view() requires (default_initializable<_Vp>
+                            && default_initializable<_Pattern>
+                            && default_initializable<iterator_t<_Vp>>)
+       = default;
 
       constexpr
       split_view(_Vp __base, _Pattern __pattern)
@@ -3229,7 +3247,7 @@ namespace views
       _Vp _M_base = _Vp();
 
     public:
-      common_view() = default;
+      common_view() requires default_initializable<_Vp> = default;
 
       constexpr explicit
       common_view(_Vp __r)
@@ -3339,7 +3357,7 @@ namespace views
                                      _M_cached_begin;
 
     public:
-      reverse_view() = default;
+      reverse_view() requires default_initializable<_Vp> = default;
 
       constexpr explicit
       reverse_view(_Vp __r)
@@ -3466,7 +3484,7 @@ namespace views
     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)
@@ -3587,7 +3605,7 @@ namespace views
            = 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)

diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index 4b389a46af7bc3257e72bb8f4a754542cb022e45..35df1a64c0e75c1bcb83ea75ab173e36cd5dc823 100644 (file)
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -453,8 +453,7 @@ _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

diff --git a/libstdc++-v3/testsuite/std/ranges/p2325.cc b/libstdc++-v3/testsuite/std/ranges/p2325.cc
new file mode 100644 (file)
index 0000000..aff3bd6
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/p2325.cc
@@ -0,0 +1,181 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// P2325R3 "Views should not be required to be default constructible"
+
+// Parts of P2325R3 are deliberately omitted in libstdc++ 10, in particular the
+// removal of default ctors for back_/front_insert_iterator, ostream_iterator,
+// ref_view and basic_istream_view/::iterator, so as to maximize backward
+// compatibility with pre-P2325R3 code.  So most static_asserts in this test fail,
+// see the xfails at the end of this file.
+
+#include <ranges>
+#include <iterator>
+#include <span>
+#include <sstream>
+#include <vector>
+#include <testsuite_iterators.h>
+
+using namespace std;
+
+template<default_initializable T> void f();
+template<typename T> requires weakly_incrementable<T> || ranges::view<T> void f();
+
+void
+test01()
+{
+  // Verify neither std::weakly_incrementable nor ranges::view require
+  // default_initializable.
+  f<int>(); // { dg-error "ambiguous" }
+}
+
+void
+test02()
+{
+  // Verify these iterators are not default constructible.
+  static_assert(!default_initializable<insert_iterator<vector<int>>>);
+  static_assert(!default_initializable<front_insert_iterator<vector<int>>>);
+  static_assert(!default_initializable<back_insert_iterator<vector<int>>>);
+  static_assert(!default_initializable<ostream_iterator<int>>);
+
+  using iter = ostream_iterator<int>;
+
+  // Verify common_iterator is conditionally default constructible.
+  static_assert(!default_initializable<common_iterator<iter, unreachable_sentinel_t>>);
+  static_assert(default_initializable<common_iterator<int*, unreachable_sentinel_t>>);
+
+  // Verify counted_iterator is conditionally default constructible.
+  static_assert(!default_initializable<counted_iterator<iter>>);
+  static_assert(default_initializable<counted_iterator<int*>>);
+}
+
+void
+test03()
+{
+  using iter = ostream_iterator<int>;
+
+  // Verify iota_view is conditionally default constructible.
+  static_assert(!default_initializable<ranges::iota_view<iter>>);
+  static_assert(!default_initializable<decltype(declval<ranges::iota_view<iter>>().begin())>);
+  static_assert(default_initializable<ranges::iota_view<int>>);
+  static_assert(default_initializable<decltype(declval<ranges::iota_view<int>>().begin())>);
+
+  // Verify subrange is conditionally default constructible.
+  static_assert(!default_initializable<ranges::subrange<iter, unreachable_sentinel_t>>);
+  static_assert(default_initializable<ranges::subrange<int*, unreachable_sentinel_t>>);
+
+  // Verify single_view is conditionally default constructible.
+  static_assert(!default_initializable<ranges::single_view<iter>>);
+  static_assert(default_initializable<ranges::single_view<int*>>);
+}
+
+void
+test04()
+{
+  // Verify basic_istream_view is not default constructible.
+  using type = ranges::basic_istream_view<int, char, char_traits<char>>;
+  static_assert(!default_initializable<type>);
+  static_assert(!default_initializable<decltype(declval<type>().begin())>);
+}
+
+void
+test05()
+{
+  // Verify ref_view is not default constructible.
+  static_assert(!default_initializable<ranges::ref_view<int[5]>>);
+}
+
+template<auto& adaptor>
+void
+test06()
+{
+  auto f1 = [] (auto) { return true; };
+  auto f2 = [i=0] (auto) { return true; };
+  static_assert(default_initializable<decltype(views::single(0) | adaptor(f1))>);
+  static_assert(!default_initializable<decltype(views::single(0) | adaptor(f2))>);
+
+  struct S { S() = delete; };
+  static_assert(!default_initializable<decltype(views::single(declval<S>()) | adaptor(f1))>);
+  static_assert(!default_initializable<decltype(views::single(declval<S>()) | adaptor(f2))>);
+}
+
+// Verify filter_view, transform_view, take_while_view and drop_while_view are
+// conditionally default constructible.
+template void test06<views::filter>();
+template void test06<views::transform>();
+template void test06<views::take_while>();
+template void test06<views::drop_while>();
+
+void
+test07()
+{
+  // Verify join_view is conditionally default constructible.
+  struct S { S() = delete; };
+  using type1 = ranges::join_view<ranges::single_view<ranges::single_view<S>>>;
+  static_assert(!default_initializable<type1>);
+  using type2 = ranges::join_view<ranges::single_view<ranges::single_view<int>>>;
+  static_assert(default_initializable<type2>);
+}
+
+void
+test08()
+{
+  // Verify split_view is conditionally default constructible.
+  using type1 = ranges::split_view<ranges::ref_view<int[2]>, ranges::single_view<int>>;
+  static_assert(!default_initializable<type1>);
+  using type2 = ranges::split_view<ranges::single_view<int>, ranges::ref_view<int[2]>>;
+  static_assert(!default_initializable<type2>);
+  using type3 = ranges::split_view<ranges::ref_view<int[2]>, ranges::ref_view<int[2]>>;
+  static_assert(!default_initializable<type3>);
+  using type4 = ranges::split_view<ranges::single_view<int>, ranges::single_view<int>>;
+  static_assert(default_initializable<type4>);
+}
+
+void
+test09()
+{
+  // Verify common_view is conditionally default constructible.
+  using type1 = ranges::common_view<ranges::iota_view<ostream_iterator<int>>>;
+  static_assert(!default_initializable<type1>);
+  using type2 = ranges::common_view<ranges::iota_view<int*>>;
+  static_assert(default_initializable<type2>);
+}
+
+void
+test10()
+{
+  // Verify reverse_view is conditionally default constructible.
+  using type1 = ranges::reverse_view<ranges::ref_view<int[2]>>;
+  static_assert(!default_initializable<type1>);
+  using type2 = ranges::reverse_view<ranges::single_view<int>>;
+  static_assert(default_initializable<type2>);
+}
+
+void
+test11()
+{
+  // Verify elements_view is conditionally default constructible.
+  using type1 = ranges::elements_view<ranges::ref_view<pair<int,int>[2]>, 0>;
+  static_assert(!default_initializable<type1>);
+  using type2 = ranges::elements_view<ranges::single_view<pair<int,int>>, 0>;
+  static_assert(default_initializable<type2>);
+}
+
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 35 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 36 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 37 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 38 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 43 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 47 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 57 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 58 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 63 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 67 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 76 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 77 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 84 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 124 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 126 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 128 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 138 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 148 }
+// { dg-bogus "static assertion failed" "" { xfail *-*-* } 158 }

diff --git a/libstdc++-v3/testsuite/std/ranges/single_view.cc b/libstdc++-v3/testsuite/std/ranges/single_view.cc
index 658e0a0a32de11519e168ebfac7492921ba2e788..484064470609412147774439861cbc7b4c3e8c4c 100644 (file)
--- a/libstdc++-v3/testsuite/std/ranges/single_view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/single_view.cc
@@ -58,9 +58,24 @@ test03()
   VERIFY(*std::ranges::begin(s3) == 'a');
 }
 
+void
+test06()
+{
+  // PR libstdc++/100475 comment #7
+  struct S {
+    S() = default;
+    S(std::initializer_list<S>) = delete;
+    S(const S&) {}
+  };
+  S obj;
+  auto x = std::views::single(obj);
+  auto y = std::views::single(std::move(obj));
+}
+
 int main()
 {
   test01();
   test02();
   test03();
+  test06();
 }

diff --git a/libstdc++-v3/testsuite/std/ranges/view.cc b/libstdc++-v3/testsuite/std/ranges/view.cc
index 72e8bcb50d74485f025c080ceb619ace1912bc6e..095a92e466fb743309224eee5e24b0d3bbe8e552 100644 (file)
--- a/libstdc++-v3/testsuite/std/ranges/view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/view.cc
@@ -31,7 +31,7 @@
 
 static_assert(std::ranges::view<std::span<int>>);
 static_assert(std::ranges::view<std::span<int, 0>>);
-static_assert(!std::ranges::view<std::span<int, 1>>);
+static_assert(std::ranges::view<std::span<int, 1>>); // Changed with P2325R3
 static_assert(std::ranges::view<std::string_view>);
 static_assert(std::ranges::view<std::experimental::string_view>);