[gcc r12-6483] libstdc++: Make copyable-box completely constexpr (LWG 3572)

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:d2dc5305d87f4e6a1b861f59164c124636e2b69d

commit r12-6483-gd2dc5305d87f4e6a1b861f59164c124636e2b69d
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jan 11 14:11:46 2022 +0000

    libstdc++: Make copyable-box completely constexpr (LWG 3572)
    
    This LWG issue was approved at the October 2021 plenary and can be
    implemented now that std::optional is fully constexpr.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/ranges (ranges::__detail::__box): Add constexpr to
            assignment operators (LWG 3572).
            * testsuite/std/ranges/adaptors/filter.cc: Check assignment of a
            view that uses copyable-box.

Diff:
---
 libstdc++-v3/include/std/ranges                    |  5 ++--
 .../testsuite/std/ranges/adaptors/filter.cc        | 28 ++++++++++++++++++++++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index c90d33c89fb..780a3633417 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -109,7 +109,8 @@ namespace ranges
 
 	// _GLIBCXX_RESOLVE_LIB_DEFECTS
 	// 3477. Simplify constraints for semiregular-box
-	__box&
+	// 3572. copyable-box should be fully constexpr
+	constexpr __box&
 	operator=(const __box& __that)
 	noexcept(is_nothrow_copy_constructible_v<_Tp>)
 	requires (!copyable<_Tp>)
@@ -124,7 +125,7 @@ namespace ranges
 	  return *this;
 	}
 
-	__box&
+	constexpr __box&
 	operator=(__box&& __that)
 	noexcept(is_nothrow_move_constructible_v<_Tp>)
 	requires (!movable<_Tp>)
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc
index b8d081e1d5e..ed5a01ca595 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/filter.cc
@@ -140,6 +140,33 @@ test06()
   static_assert(!requires { views::all | filter; });
 }
 
+constexpr bool
+test07()
+{
+  struct Pred
+  {
+    constexpr Pred() { }
+    constexpr Pred(const Pred&) { }
+    constexpr Pred(Pred&&) { }
+    // These make it non-copyable, so non-copyable-box<Pred> will provide
+    // assignment.
+    Pred& operator=(const Pred&) = delete;
+    Pred& operator=(Pred&&) = delete;
+
+    bool operator()(int i) const { return i < 10; }
+  };
+
+  int i = 0;
+  ranges::filter_view v(views::single(i), Pred{});
+  // LWG 3572. copyable-box should be fully constexpr
+  v = v;
+  v = std::move(v);
+
+  return true;
+}
+
+static_assert( test07() );
+
 int
 main()
 {
@@ -150,4 +177,5 @@ main()
   test05<forward_iterator_wrapper>();
   test05<random_access_iterator_wrapper>();
   test06();
+  test07();
 }