libstdc++: Implement LWG 3549 changes to ranges::enable_view

This patch also reverts r11-3504 since that workaround is now obsolete
after this resolution.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (view_interface): Forward declare.
	(__detail::__is_derived_from_view_interface_fn): Declare.
	(__detail::__is_derived_from_view_interface): Define as per LWG 3549.
	(enable_view): Adjust as per LWG 3549.
	* include/bits/ranges_util.h (view_interface): Don't derive from
	view_base.
	* include/std/ranges (filter_view): Revert r11-3504 change.
	(transform_view): Likewise.
	(take_view): Likewise.
	(take_while_view): Likewise.
	(drop_view): Likewise.
	(drop_while_view): Likewise.
	(join_view): Likewise.
	(lazy_split_view): Likewise.
	(split_view): Likewise.
	(reverse_view): Likewise.
	* testsuite/std/ranges/adaptors/sizeof.cc: Update expected sizes.
	* testsuite/std/ranges/view.cc (test_view::test_view): Remove
	this default ctor since views no longer need to be default initable.
	(test01): New test.

5 files changed, 70 insertions, 32 deletions

diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
index 01d0c35..7801b2f 100644
--- a/libstdc++-v3/include/bits/ranges_base.h
+++ b/libstdc++-v3/include/bits/ranges_base.h
@@ -614,12 +614,31 @@ namespace ranges
   template<sized_range _Range>
     using range_size_t = decltype(ranges::size(std::declval<_Range&>()));
 
+  template<typename _Derived>
+    requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
+    class view_interface; // defined in <bits/ranges_util.h>
+
+  namespace __detail
+  {
+    template<typename _Tp, typename _Up>
+      requires (!same_as<_Tp, view_interface<_Up>>)
+      void __is_derived_from_view_interface_fn(const _Tp&,
+					       const view_interface<_Up>&); // not defined
+
+    // Returns true iff _Tp has exactly one public base class that's a
+    // specialization of view_interface.
+    template<typename _Tp>
+      concept __is_derived_from_view_interface
+	= requires (_Tp __t) { __is_derived_from_view_interface_fn(__t, __t); };
+  }
+
   /// [range.view] The ranges::view_base type.
   struct view_base { };
 
   /// [range.view] The ranges::enable_view boolean.
   template<typename _Tp>
-    inline constexpr bool enable_view = derived_from<_Tp, view_base>;
+    inline constexpr bool enable_view = derived_from<_Tp, view_base>
+      || __detail::__is_derived_from_view_interface<_Tp>;
 
   /// [range.view] The ranges::view concept.
   template<typename _Tp>
diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
index 7e7b958..aaa7e8c 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -61,7 +61,7 @@ namespace ranges
   /// The ranges::view_interface class template
   template<typename _Derived>
     requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
-    class view_interface : public view_base
+    class view_interface
     {
     private:
       constexpr _Derived& _M_derived() noexcept
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 18bd087..6c99892 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1483,9 +1483,9 @@ namespace views::__adaptor
 	{ return __y.__equal(__x); }
       };
 
+      _Vp _M_base = _Vp();
       [[no_unique_address]] __detail::__box<_Pred> _M_pred;
       [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
-      _Vp _M_base = _Vp();
 
     public:
       filter_view() requires (default_initializable<_Vp>
@@ -1494,7 +1494,7 @@ namespace views::__adaptor
 
       constexpr
       filter_view(_Vp __base, _Pred __pred)
-	: _M_pred(std::move(__pred)), _M_base(std::move(__base))
+	: _M_base(std::move(__base)), _M_pred(std::move(__pred))
       { }
 
       constexpr _Vp
@@ -1850,8 +1850,8 @@ namespace views::__adaptor
 	  friend _Sentinel<!_Const>;
 	};
 
-      [[no_unique_address]] __detail::__box<_Fp> _M_fun;
       _Vp _M_base = _Vp();
+      [[no_unique_address]] __detail::__box<_Fp> _M_fun;
 
     public:
       transform_view() requires (default_initializable<_Vp>
@@ -1860,7 +1860,7 @@ namespace views::__adaptor
 
       constexpr
       transform_view(_Vp __base, _Fp __fun)
-	: _M_fun(std::move(__fun)), _M_base(std::move(__base))
+	: _M_base(std::move(__base)), _M_fun(std::move(__fun))
       { }
 
       constexpr _Vp
@@ -1987,15 +1987,15 @@ namespace views::__adaptor
 	  friend _Sentinel<!_Const>;
 	};
 
-      range_difference_t<_Vp> _M_count = 0;
       _Vp _M_base = _Vp();
+      range_difference_t<_Vp> _M_count = 0;
 
     public:
       take_view() requires default_initializable<_Vp> = default;
 
       constexpr
       take_view(_Vp base, range_difference_t<_Vp> __count)
-	: _M_count(std::move(__count)), _M_base(std::move(base))
+	: _M_base(std::move(base)), _M_count(std::move(__count))
       { }
 
       constexpr _Vp
@@ -2171,8 +2171,8 @@ namespace views::__adaptor
 	  friend _Sentinel<!_Const>;
 	};
 
-      [[no_unique_address]] __detail::__box<_Pred> _M_pred;
       _Vp _M_base = _Vp();
+      [[no_unique_address]] __detail::__box<_Pred> _M_pred;
 
     public:
       take_while_view() requires (default_initializable<_Vp>
@@ -2181,7 +2181,7 @@ namespace views::__adaptor
 
       constexpr
       take_while_view(_Vp base, _Pred __pred)
-	: _M_pred(std::move(__pred)), _M_base(std::move(base))
+	: _M_base(std::move(base)), _M_pred(std::move(__pred))
       { }
 
       constexpr _Vp
@@ -2252,8 +2252,8 @@ namespace views::__adaptor
     class drop_view : public view_interface<drop_view<_Vp>>
     {
     private:
-      range_difference_t<_Vp> _M_count = 0;
       _Vp _M_base = _Vp();
+      range_difference_t<_Vp> _M_count = 0;
 
       // ranges::next(begin(base), count, end(base)) is O(1) if _Vp satisfies
       // both random_access_range and sized_range. Otherwise, cache its result.
@@ -2269,7 +2269,7 @@ namespace views::__adaptor
 
       constexpr
       drop_view(_Vp __base, range_difference_t<_Vp> __count)
-	: _M_count(__count), _M_base(std::move(__base))
+	: _M_base(std::move(__base)), _M_count(__count)
       { __glibcxx_assert(__count >= 0); }
 
       constexpr _Vp
@@ -2376,9 +2376,9 @@ namespace views::__adaptor
     class drop_while_view : public view_interface<drop_while_view<_Vp, _Pred>>
     {
     private:
+      _Vp _M_base = _Vp();
       [[no_unique_address]] __detail::__box<_Pred> _M_pred;
       [[no_unique_address]] __detail::_CachedPosition<_Vp> _M_cached_begin;
-      _Vp _M_base = _Vp();
 
     public:
       drop_while_view() requires (default_initializable<_Vp>
@@ -2387,7 +2387,7 @@ namespace views::__adaptor
 
       constexpr
       drop_while_view(_Vp __base, _Pred __pred)
-	: _M_pred(std::move(__pred)), _M_base(std::move(__base))
+	: _M_base(std::move(__base)), _M_pred(std::move(__pred))
       { }
 
       constexpr _Vp
@@ -2723,9 +2723,9 @@ namespace views::__adaptor
 	  friend _Sentinel<!_Const>;
 	};
 
+      _Vp _M_base = _Vp();
       [[no_unique_address]]
 	__detail::__non_propagating_cache<remove_cv_t<_InnerRange>> _M_inner;
-      _Vp _M_base = _Vp();
 
     public:
       join_view() requires default_initializable<_Vp> = default;
@@ -3161,12 +3161,12 @@ namespace views::__adaptor
 	  { ranges::iter_swap(__x._M_i_current(), __y._M_i_current()); }
 	};
 
+      _Vp _M_base = _Vp();
       _Pattern _M_pattern = _Pattern();
       // XXX: _M_current is "present only if !forward_range<V>"
       [[no_unique_address]]
 	__detail::__maybe_present_t<!forward_range<_Vp>,
 	  __detail::__non_propagating_cache<iterator_t<_Vp>>> _M_current;
-      _Vp _M_base = _Vp();
 
 
     public:
@@ -3176,7 +3176,7 @@ namespace views::__adaptor
 
       constexpr
       lazy_split_view(_Vp __base, _Pattern __pattern)
-	: _M_pattern(std::move(__pattern)), _M_base(std::move(__base))
+	: _M_base(std::move(__base)), _M_pattern(std::move(__pattern))
       { }
 
       template<input_range _Range>
@@ -3184,8 +3184,8 @@ namespace views::__adaptor
 	  && constructible_from<_Pattern, single_view<range_value_t<_Range>>>
 	constexpr
 	lazy_split_view(_Range&& __r, range_value_t<_Range> __e)
-	  : _M_pattern(views::single(std::move(__e))),
-	    _M_base(views::all(std::forward<_Range>(__r)))
+	  : _M_base(views::all(std::forward<_Range>(__r))),
+	    _M_pattern(views::single(std::move(__e)))
 	{ }
 
       constexpr _Vp
@@ -3283,9 +3283,9 @@ namespace views::__adaptor
   class split_view : public view_interface<split_view<_Vp, _Pattern>>
   {
   private:
+    _Vp _M_base = _Vp();
     _Pattern _M_pattern = _Pattern();
     __detail::__non_propagating_cache<subrange<iterator_t<_Vp>>> _M_cached_begin;
-    _Vp _M_base = _Vp();
 
     struct _Iterator;
     struct _Sentinel;
@@ -3297,8 +3297,7 @@ namespace views::__adaptor
 
     constexpr
     split_view(_Vp __base, _Pattern __pattern)
-      : _M_pattern(std::move(__pattern)),
-	_M_base(std::move(__base))
+      : _M_base(std::move(__base)), _M_pattern(std::move(__pattern))
     { }
 
     template<forward_range _Range>
@@ -3306,8 +3305,8 @@ namespace views::__adaptor
 	&& constructible_from<_Pattern, single_view<range_value_t<_Range>>>
     constexpr
     split_view(_Range&& __r, range_value_t<_Range> __e)
-      : _M_pattern(views::single(std::move(__e))),
-	_M_base(views::all(std::forward<_Range>(__r)))
+      : _M_base(views::all(std::forward<_Range>(__r))),
+	_M_pattern(views::single(std::move(__e)))
     { }
 
     constexpr _Vp
@@ -3630,11 +3629,11 @@ namespace views::__adaptor
 				  && sized_sentinel_for<sentinel_t<_Vp>,
 							iterator_t<_Vp>>);
 
+      _Vp _M_base = _Vp();
       [[no_unique_address]]
 	__detail::__maybe_present_t<_S_needs_cached_begin,
 				    __detail::_CachedPosition<_Vp>>
 				      _M_cached_begin;
-      _Vp _M_base = _Vp();
 
     public:
       reverse_view() requires default_initializable<_Vp> = default;
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc
index 219e2a6..61524d4 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/sizeof.cc
@@ -42,11 +42,11 @@ static_assert(sizeof(ranges::drop_while_view<V, decltype(&pred_f)>) == 4*ptr);
 static_assert(sizeof(ranges::transform_view<V, decltype(&func_f)>) == 3*ptr);
 
 static_assert(sizeof(ranges::filter_view<V, decltype(pred_l)>) == 3*ptr);
-static_assert(sizeof(ranges::take_while_view<V, decltype(pred_l)>) == 3*ptr);
+static_assert(sizeof(ranges::take_while_view<V, decltype(pred_l)>) == 2*ptr);
 static_assert(sizeof(ranges::drop_while_view<V, decltype(pred_l)>) == 3*ptr);
-static_assert(sizeof(ranges::transform_view<V, decltype(func_l)>) == 3*ptr);
+static_assert(sizeof(ranges::transform_view<V, decltype(func_l)>) == 2*ptr);
 
 static_assert(sizeof(ranges::lazy_split_view<V, std::string_view>) == 4*ptr);
 
 static_assert
- (sizeof(ranges::reverse_view<ranges::filter_view<V, decltype(pred_l)>>) == 4*ptr);
+ (sizeof(ranges::reverse_view<ranges::filter_view<V, decltype(pred_l)>>) == 3*ptr);
diff --git a/libstdc++-v3/testsuite/std/ranges/view.cc b/libstdc++-v3/testsuite/std/ranges/view.cc
index dd82582..98a162e 100644
--- a/libstdc++-v3/testsuite/std/ranges/view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/view.cc
@@ -52,10 +52,7 @@ static_assert(!std::ranges::view<__gnu_test::test_random_access_range<int>>);
 template<typename T>
 struct test_view
 : __gnu_test::test_random_access_range<T>, std::ranges::view_base
-{
-  // views must be default-initializable:
-  test_view() : __gnu_test::test_random_access_range<T>(nullptr, nullptr) { }
-};
+{ };
 
 static_assert(std::ranges::view<test_view<int>>);
 
@@ -63,3 +60,26 @@ template<>
 constexpr bool std::ranges::enable_view<test_view<long>> = false;
 
 static_assert(!std::ranges::view<test_view<long>>);
+
+void
+test01()
+{
+  // Verify LWG 3549 changes to ranges::enable_view.
+  using std::ranges::view_interface;
+
+  struct v1
+    : __gnu_test::test_random_access_range<int>, view_interface<v1> { };
+  static_assert(!std::derived_from<v1, std::ranges::view_base>);
+  static_assert(std::ranges::enable_view<v1>);
+
+  struct v2 : v1, view_interface<v2> { };
+  static_assert(!std::derived_from<v2, std::ranges::view_base>);
+  static_assert(!std::ranges::enable_view<v2>);
+
+  struct v3 : __gnu_test::test_random_access_range<int> { };
+  static_assert(!std::derived_from<v3, std::ranges::view_base>);
+  static_assert(!std::ranges::enable_view<v3>);
+
+  struct v4 { };
+  static_assert(!std::ranges::enable_view<view_interface<v4>>);
+}