[libc++][ranges] LWG4096: views::iota(views::iota(0)) should be rejected (#152855)

Fixes #105352

Author: H-G-Hristov

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",""

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>
@@ -374,10 +375,10 @@ namespace views {
 namespace __iota {
 struct __fn {
   template <class _Start>
+    requires(requires(_Start __s) { ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__s)); })
   _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));
+      noexcept(noexcept(ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__start)))) {
+    return ranges::iota_view<decay_t<_Start>>(std::forward<_Start>(__start));
   }
 
   template <class _Start, class _BoundSentinel>

libcxx/test/std/ranges/range.factories/range.iota.view/views_iota.pass.cpp

@@ -46,6 +46,9 @@ constexpr void testType(U u) {
 
 struct X {};
 
+template <typename IntT>
+concept CanDoubleWrap = requires(IntT i) { std::views::iota(std::views::iota(i)); };
+
 constexpr bool test() {
   testType<SomeInt>(SomeInt(10));
   testType<SomeInt>(IntComparableWith(SomeInt(10)));
@@ -68,6 +71,15 @@ constexpr bool test() {
   {
     static_assert(std::same_as<decltype(std::views::iota), decltype(std::ranges::views::iota)>);
   }
+  { // LWG4096: views::iota(views::iota(0)) should be rejected
+    static_assert(!CanDoubleWrap<int>);
+    static_assert(!CanDoubleWrap<SomeInt>);
+
+    static_assert(!std::is_invocable_v<decltype(std::views::iota), decltype(std::views::iota(0))>);
+    static_assert(!std::is_invocable_v<decltype(std::views::iota), decltype(std::views::iota(82))>);
+    static_assert(!std::is_invocable_v<decltype(std::views::iota), decltype(std::views::iota(SomeInt(0)))>);
+    static_assert(!std::is_invocable_v<decltype(std::views::iota), decltype(std::views::iota(SomeInt(94)))>);
+  }
 
   return true;
 }