[libc++][ranges] Reject non-class types in ranges::to

libcxx/include/__ranges/to.h

@@ -26,7 +26,9 @@
 #include <__ranges/size.h>
 #include <__ranges/transform_view.h>
 #include <__type_traits/add_pointer.h>
+#include <__type_traits/is_class.h>
 #include <__type_traits/is_const.h>
+#include <__type_traits/is_union.h>
 #include <__type_traits/is_volatile.h>
 #include <__type_traits/type_identity.h>
 #include <__utility/declval.h>
@@ -81,7 +83,7 @@ template <class _Container, input_range _Range, class... _Args>
   static_assert(!is_const_v<_Container>, "The target container cannot be const-qualified, please remove the const");
   static_assert(
       !is_volatile_v<_Container>, "The target container cannot be volatile-qualified, please remove the volatile");
-
+  static_assert(is_class_v<_Container> || is_union_v<_Container>, "The target must be a class type or union type");
   // First see if the non-recursive case applies -- the conversion target is either:
   // - a range with a convertible value type;
   // - a non-range type which might support being created from the input argument(s) (e.g. an `optional`).
@@ -208,7 +210,7 @@ template <class _Container, class... _Args>
   static_assert(!is_const_v<_Container>, "The target container cannot be const-qualified, please remove the const");
   static_assert(
       !is_volatile_v<_Container>, "The target container cannot be volatile-qualified, please remove the volatile");
-
+  static_assert(is_class_v<_Container> || is_union_v<_Container>, "The target must be a class type or union type");
   auto __to_func = []<input_range _Range, class... _Tail>(_Range&& __range, _Tail&&... __tail) static
     requires requires { //
       /**/ ranges::to<_Container>(std::forward<_Range>(__range), std::forward<_Tail>(__tail)...);

libcxx/test/libcxx/ranges/range.utility/range.utility.conv/to.verify.cpp

@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// Test that the "mandates" requirements on the given class are checked using `static_assert`.
+#include <ranges>
+void ff() {}
+void test() {
+  struct C {
+    int member;
+    int f() { return 0; }
+  };
+
+  enum color { red, green, blue };
+  using member_func_ptr = decltype(&C::f);
+  using member_ptr      = decltype(&C::member);
+  using func_ptr        = decltype(&ff);
+  using func_t          = decltype(ff);
+
+  struct R {
+    int* begin() const { return nullptr; };
+    int* end() const { return nullptr; };
+
+    operator int() const { return 0; }
+    operator int*() const { return nullptr; }
+    operator func_ptr() const { return nullptr; }
+    operator member_func_ptr() const { return nullptr; }
+    operator member_ptr() const { return nullptr; }
+    operator color() const { return color::red; }
+  };
+  (void)std::ranges::to<int>(
+      R{}); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)(R{} | std::ranges::to<
+                   int>()); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)std::ranges::to<int*>(
+      R{}); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)(R{} | std::ranges::to<
+                   int*>()); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)std::ranges::to<func_ptr>(
+      R{}); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)(R{} |
+         std::ranges::to<
+             func_ptr>()); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+
+  (void)std::ranges::to<member_ptr>(
+      R{}); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)(R{} |
+         std::ranges::to<
+             member_ptr>()); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+
+  (void)std::ranges::to<func_t>(
+      R{}); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)(R{} | std::ranges::to<
+                   func_t>()); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+
+  (void)std::ranges::to<void>(
+      R{}); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)(R{} | std::ranges::to<
+                   void>()); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  //expected-error-re@*:* {{static assertion failed{{.*}}ranges::to: unable to convert to the given container type.}}
+
+  (void)std::ranges::to<color>(
+      R{}); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}
+  (void)(R{} | std::ranges::to<
+                   color>()); //expected-error-re@*:* {{static assertion failed{{.*}}The target must be a class type}}                  
+}