[libcxx-commits] [libcxx] [libc++][ranges] LWG4096: `views::iota(views::iota(0))` should be rejected (PR #152855)

[Hristo Hristov via libcxx-commits]

https://github.com/H-G-Hristov created https://github.com/llvm/llvm-project/pull/152855

Fixes: #105352

>From e66aca58e43cfb9fcb55bc4025b12e6b57031f72 Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm at gmail.com>
Date: Sat, 9 Aug 2025 11:45:35 +0300
Subject: [PATCH] [libc++][ranges] LWG4096: `views::iota(views::iota(0))`
 should be rejected

Fixes: #105352
---
 libcxx/docs/Status/Cxx2cIssues.csv            |  2 +-
 libcxx/include/__ranges/iota_view.h           |  3 +-
 .../range.iota.view/views_iota.verify.cpp     | 29 +++++++++++++++++++
 3 files changed, 32 insertions(+), 2 deletions(-)
 create mode 100644 libcxx/test/std/ranges/range.factories/range.iota.view/views_iota.verify.cpp

diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 6fcb2f3c78cfc..9b61c921c8db4 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -71,7 +71,7 @@
 "`LWG4079 <https://wg21.link/LWG4079>`__","Missing Preconditions in ``concat_view::iterator``\`s conversion constructor","2024-06 (St. Louis)","","",""
 "`LWG4082 <https://wg21.link/LWG4082>`__","``views::concat(r)`` is well-formed when ``r`` is an ``output_range``","2024-06 (St. Louis)","","",""
 "`LWG4083 <https://wg21.link/LWG4083>`__","``views::as_rvalue`` should reject non-input ranges","2024-06 (St. Louis)","","",""
-"`LWG4096 <https://wg21.link/LWG4096>`__","``views::iota(views::iota(0))`` should be rejected","2024-06 (St. Louis)","","",""
+"`LWG4096 <https://wg21.link/LWG4096>`__","``views::iota(views::iota(0))`` should be rejected","2024-06 (St. Louis)","|Complete|","22",""
 "`LWG4098 <https://wg21.link/LWG4098>`__","``views::adjacent<0>`` should reject non-forward ranges","2024-06 (St. Louis)","","",""
 "`LWG4105 <https://wg21.link/LWG4105>`__","``ranges::ends_with``\`s Returns misses difference casting","2024-06 (St. Louis)","","",""
 "`LWG4106 <https://wg21.link/LWG4106>`__","``basic_format_args`` should not be default-constructible","2024-06 (St. Louis)","|Complete|","19",""
diff --git a/libcxx/include/__ranges/iota_view.h b/libcxx/include/__ranges/iota_view.h
index 4b84585258b91..79a450a9ce40c 100644
--- a/libcxx/include/__ranges/iota_view.h
+++ b/libcxx/include/__ranges/iota_view.h
@@ -30,6 +30,7 @@
 #include <__ranges/movable_box.h>
 #include <__ranges/view_interface.h>
 #include <__type_traits/conditional.h>
+#include <__type_traits/decay.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/make_unsigned.h>
 #include <__type_traits/type_identity.h>
@@ -377,7 +378,7 @@ struct __fn {
   _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Start&& __start) const
       noexcept(noexcept(ranges::iota_view(std::forward<_Start>(__start))))
           -> decltype(ranges::iota_view(std::forward<_Start>(__start))) {
-    return ranges::iota_view(std::forward<_Start>(__start));
+    return ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__start));
   }
 
   template <class _Start, class _BoundSentinel>
diff --git a/libcxx/test/std/ranges/range.factories/range.iota.view/views_iota.verify.cpp b/libcxx/test/std/ranges/range.factories/range.iota.view/views_iota.verify.cpp
new file mode 100644
index 0000000000000..9f1879968eebc
--- /dev/null
+++ b/libcxx/test/std/ranges/range.factories/range.iota.view/views_iota.verify.cpp
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++20
+
+// views::iota
+
+#include <ranges>
+
+#include "types.h"
+
+void test() {
+  // LWG4096
+  {
+    [[maybe_unused]] auto i1 = std::views::iota(0); // OK
+    [[maybe_unused]] auto i2 = std::views::iota(std::views::iota(0));
+    // expected-error-re@*:* {{constraints not satisfied for class template 'iota_view'{{.*}}}}
+  }
+  {
+    [[maybe_unused]] auto i1 = std::views::iota(SomeInt(0)); // OK
+    [[maybe_unused]] auto i2 = std::views::iota(std::views::iota(SomeInt(0)));
+    //expected-error-re@*:* {{constraints not satisfied for class template 'iota_view'{{.*}}}}
+  }
+}