From 16592022bab65093d75b6700b1114b052f7cd42e Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Thu, 16 Dec 2021 21:46:59 -0800
Subject: [PATCH 1/2] subrange `get<0>` requires a copyable iterator

per LWG-3589.

Fixes #2399
---
 stl/inc/xutility                              |  4 +-
 .../test.compile.pass.cpp                     | 37 ++++++++++++++++++-
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/stl/inc/xutility b/stl/inc/xutility
index a8f41b14b5..8ff0e14410 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -3487,7 +3487,7 @@ namespace ranges {
 
     // clang-format off
     template <size_t _Idx, class _It, class _Se, subrange_kind _Ki>
-        requires (_Idx < 2)
+        requires ((_Idx  == 0 && copyable<_It>) || _Idx == 1)
     _NODISCARD constexpr auto get(const subrange<_It, _Se, _Ki>& _Val) {
         if constexpr (_Idx == 0) {
             return _Val.begin();
@@ -3497,7 +3497,7 @@ namespace ranges {
     }
 
     template <size_t _Idx, class _It, class _Se, subrange_kind _Ki>
-        requires (_Idx < 2)
+        requires ((_Idx  == 0 && copyable<_It>) || _Idx == 1)
     _NODISCARD constexpr auto get(subrange<_It, _Se, _Ki>&& _Val) {
         if constexpr (_Idx == 0) {
             return _Val.begin();
diff --git a/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp b/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp
index be0cbaa1d7..f1ff556d15 100644
--- a/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp
+++ b/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp
@@ -4,8 +4,8 @@
 // Covers ranges::view_interface and ranges::subrange
 
 #include <cassert>
-#include <concepts>
 #include <forward_list>
+#include <istream>
 #include <list>
 #include <ranges>
 #include <string_view>
@@ -24,6 +24,7 @@ using std::output_iterator_tag, std::input_iterator_tag, std::forward_iterator_t
 int main() {} // COMPILE-ONLY
 
 void test_LWG_3470() {
+    // LWG-3470 relaxed the "convertible-to-non-slicing" requirements to allow this non-slicing case
     int a[]                 = {1, 2, 3};
     int* b[]                = {&a[2], &a[0], &a[1]};
     [[maybe_unused]] auto c = std::ranges::subrange<const int* const*>(b);
@@ -1483,3 +1484,37 @@ namespace test_subrange {
     STATIC_ASSERT(test_tuple<subrange<int*, std::unreachable_sentinel_t, subrange_kind::sized>>());
     STATIC_ASSERT(test_tuple<subrange<int*, std::unreachable_sentinel_t, subrange_kind::unsized>>());
 } // namespace test_subrange
+
+namespace test_lwg_3589 {
+    // LWG-3589 added a Constraint to std::get<0>(subrange) to require the iterator type to be copyable
+    template <class T, size_t I>
+    concept CanGet = requires {
+        std::get<I>(std::declval<T>());
+    };
+
+    template <class T, size_t I>
+    concept CanRangesGet = requires {
+        ranges::get<I>(std::declval<T>());
+    };
+
+    template <class I, class S>
+    constexpr bool test() {
+        using ranges::subrange;
+
+        STATIC_ASSERT(std::input_iterator<I>);
+        STATIC_ASSERT(std::sentinel_for<S, I>);
+
+        STATIC_ASSERT(CanGet<subrange<I, S>, 0> == std::copyable<I>);
+        STATIC_ASSERT(CanGet<subrange<I, S>, 1>);
+        STATIC_ASSERT(!CanGet<subrange<I, S>, 2>);
+
+        STATIC_ASSERT(CanRangesGet<subrange<I, S>, 0> == std::copyable<I>);
+        STATIC_ASSERT(CanRangesGet<subrange<I, S>, 1>);
+        STATIC_ASSERT(!CanRangesGet<subrange<I, S>, 2>);
+
+        return true;
+    }
+
+    STATIC_ASSERT(test<int*, int*>());
+    STATIC_ASSERT(test<ranges::iterator_t<ranges::istream_view<int>>, ranges::sentinel_t<ranges::istream_view<int>>>());
+} // namespace test_lwg_3589

From a57013385d13cb8a4e90218165ba65b8eade4c56 Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Sat, 18 Dec 2021 11:44:19 -0800
Subject: [PATCH 2/2] Implement LWG-3589 _correctly_

---
 stl/inc/xutility                                    |  4 ++--
 .../P0896R4_ranges_subrange/test.compile.pass.cpp   | 13 ++++++++++---
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/stl/inc/xutility b/stl/inc/xutility
index 8ff0e14410..9d455e5378 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -3487,7 +3487,7 @@ namespace ranges {
 
     // clang-format off
     template <size_t _Idx, class _It, class _Se, subrange_kind _Ki>
-        requires ((_Idx  == 0 && copyable<_It>) || _Idx == 1)
+        requires ((_Idx == 0 && copyable<_It>) || _Idx == 1)
     _NODISCARD constexpr auto get(const subrange<_It, _Se, _Ki>& _Val) {
         if constexpr (_Idx == 0) {
             return _Val.begin();
@@ -3497,7 +3497,7 @@ namespace ranges {
     }
 
     template <size_t _Idx, class _It, class _Se, subrange_kind _Ki>
-        requires ((_Idx  == 0 && copyable<_It>) || _Idx == 1)
+        requires (_Idx < 2)
     _NODISCARD constexpr auto get(subrange<_It, _Se, _Ki>&& _Val) {
         if constexpr (_Idx == 0) {
             return _Val.begin();
diff --git a/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp b/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp
index f1ff556d15..fe9ad49845 100644
--- a/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp
+++ b/tests/std/tests/P0896R4_ranges_subrange/test.compile.pass.cpp
@@ -1486,7 +1486,7 @@ namespace test_subrange {
 } // namespace test_subrange
 
 namespace test_lwg_3589 {
-    // LWG-3589 added a Constraint to std::get<0>(subrange) to require the iterator type to be copyable
+    // LWG-3589 added a Constraint to std::get<0>(const subrange&) to require the iterator type to be copyable
     template <class T, size_t I>
     concept CanGet = requires {
         std::get<I>(std::declval<T>());
@@ -1504,17 +1504,24 @@ namespace test_lwg_3589 {
         STATIC_ASSERT(std::input_iterator<I>);
         STATIC_ASSERT(std::sentinel_for<S, I>);
 
-        STATIC_ASSERT(CanGet<subrange<I, S>, 0> == std::copyable<I>);
+        STATIC_ASSERT(CanGet<const subrange<I, S>&, 0> == std::copyable<I>);
+        STATIC_ASSERT(CanGet<const subrange<I, S>&, 1>);
+        STATIC_ASSERT(!CanGet<const subrange<I, S>&, 2>);
+        STATIC_ASSERT(CanGet<subrange<I, S>, 0>);
         STATIC_ASSERT(CanGet<subrange<I, S>, 1>);
         STATIC_ASSERT(!CanGet<subrange<I, S>, 2>);
 
-        STATIC_ASSERT(CanRangesGet<subrange<I, S>, 0> == std::copyable<I>);
+        STATIC_ASSERT(CanRangesGet<const subrange<I, S>&, 0> == std::copyable<I>);
+        STATIC_ASSERT(CanRangesGet<const subrange<I, S>&, 1>);
+        STATIC_ASSERT(!CanRangesGet<const subrange<I, S>&, 2>);
+        STATIC_ASSERT(CanRangesGet<subrange<I, S>, 0>);
         STATIC_ASSERT(CanRangesGet<subrange<I, S>, 1>);
         STATIC_ASSERT(!CanRangesGet<subrange<I, S>, 2>);
 
         return true;
     }
 
+    // Validate with a copyable iterator type, and with a move-only iterator type
     STATIC_ASSERT(test<int*, int*>());
     STATIC_ASSERT(test<ranges::iterator_t<ranges::istream_view<int>>, ranges::sentinel_t<ranges::istream_view<int>>>());
 } // namespace test_lwg_3589