[libc++] Define `namespace views` in its own detail header.

Discovered in the comments on D118748: we would like this namespace
to exist anytime Ranges exists, regardless of whether concepts syntax
is supported. Also, we'd like to fully granularize the <ranges> header,
which means not putting any loose declarations at the top level.

Differential Revision: https://reviews.llvm.org/D118809

libcxx/include/CMakeLists.txt

@@ -351,6 +351,7 @@ set(files
   __ranges/take_view.h
   __ranges/transform_view.h
   __ranges/view_interface.h
+  __ranges/views.h
   __split_buffer
   __std_stream
   __string

libcxx/include/__ranges/views.h

@@ -0,0 +1,35 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___RANGES_VIEWS
+#define _LIBCPP___RANGES_VIEWS
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+namespace ranges {
+
+namespace views { }
+
+} // namespace ranges
+
+namespace views = ranges::views;
+
+#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___RANGES_VIEWS

libcxx/include/module.modulemap

@@ -817,6 +817,7 @@ module std [system] {
         export functional.__functional.perfect_forward
       }
       module view_interface         { private header "__ranges/view_interface.h" }
+      module views                  { private header "__ranges/views.h" }
     }
   }
   module ratio {

libcxx/include/ranges

@@ -258,6 +258,7 @@ namespace std {
 #include <__ranges/take_view.h>
 #include <__ranges/transform_view.h>
 #include <__ranges/view_interface.h>
+#include <__ranges/views.h>
 #include <compare>          // Required by the standard.
 #include <initializer_list> // Required by the standard.
 #include <iterator>         // Required by the standard.
@@ -267,16 +268,6 @@ namespace std {
 #  pragma GCC system_header
 #endif
 
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
-
-namespace views = ranges::views;
-
-#endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
-
-_LIBCPP_END_NAMESPACE_STD
-
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 
 #endif // _LIBCPP_RANGES

libcxx/test/libcxx/diagnostics/detail.headers/ranges/views.module.verify.cpp

@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__ranges/views.h'}}
+#include <__ranges/views.h>