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

[• Quuxplusone]

Summary

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.

(Essentially NFC.)

[• Quuxplusone]

Update cmakelists and modulemap.

[• Quuxplusone]

Arglebargle. Close namespace std after opening it.

[ldionne]

LGTM, but I think we can pick a better name for the header.

libcxx/include/CMakeLists.txt

353 ↗

(On Diff #405312)

views_namespace.h or namespace_views.h? Just views.h makes it look like it is an umbrella header for including all views at once.

[• Quuxplusone]

libcxx/include/CMakeLists.txt

353 ↗

(On Diff #405312)

__ranges/namespace_views.h wouldn't be the end of the world, but it would be oddly inconsistent with the usual convention of naming the header after the library name it defines. The header that defines the-thing-named-views should be named views.h, IMO.
(This is our convention with the notable exception of __ranges/access.h, which I've tried to disentangle before and found it's sadly really difficult.)

[ldionne]

libcxx/include/CMakeLists.txt

353 ↗

(On Diff #405312)

Okay, I'm neutral then. Go by your preference.

[• Quuxplusone]

#include <__config> (I should add a linter test for this line as well!)
If CI is green, imma land this.

[dim]

It's a pity this one didn't make it into 14.0.0, as you now can't even include <ranges> without immediately getting an error:

% cat test-ranges.cpp
#include <ranges>

% c++ -c test-ranges.cpp
In file included from test-ranges.cpp:1:
/usr/include/c++/v1/ranges:240:19: error: use of undeclared identifier 'ranges'
namespace views = ranges::views;
                  ^
1 error generated.

This should definitely go into 14.0.1, then?

[ldionne]

In D118809#3437128, @dim wrote:

It's a pity this one didn't make it into 14.0.0, as you now can't even include <ranges> without immediately getting an error:

% cat test-ranges.cpp
#include <ranges>

% c++ -c test-ranges.cpp
In file included from test-ranges.cpp:1:
/usr/include/c++/v1/ranges:240:19: error: use of undeclared identifier 'ranges'
namespace views = ranges::views;
                  ^
1 error generated.

This should definitely go into 14.0.1, then?

Sorry, this missed 14.0.1 too, but I just cherry-picked it to the release/14.x branch:

[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

(cherry picked from commit 44cdca37c01a58da94087be8ebd0ee2bd2ba724e)

libcxx/include/__ranges/views.h

+// -*- 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
+
+#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)
+
+#endif // _LIBCPP___RANGES_VIEWS
+

libcxx/include/ranges

 #include <__ranges/join_view.h>
 #include <__ranges/ref_view.h>
 #include <__ranges/reverse_view.h>
 #include <__ranges/single_view.h>
 #include <__ranges/size.h>
 #include <__ranges/subrange.h>
 #include <__ranges/take_view.h>
 #include <__ranges/transform_view.h>
+#include <__ranges/views.h>
 #include <__ranges/view_interface.h>
 #include <compare>          // Required by the standard.
 #include <initializer_list> // Required by the standard.
 #include <iterator>         // Required by the standard.
 #include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #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

+//===----------------------------------------------------------------------===//
+//
+// 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>