[libc++][modules] Introduce a forward-declaration for std::byte

[ldionne]

We need a forward-declaration so that we can know about std::byte from some type traits without having to include std::byte's definition, which (circularly) depends back on type traits.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

We need a forward-declaration so that we can know about std::byte from some type traits without having to include std::byte's definition, which (circularly) depends back on type traits.

---

Full diff: https://github.com/llvm/llvm-project/pull/107402.diff

6 Files Affected:

• (modified) libcxx/include/CMakeLists.txt (+1)
• (modified) libcxx/include/__cstddef/byte.h (+1)
• (added) libcxx/include/__fwd/byte.h (+26)
• (modified) libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h (+1-1)
• (modified) libcxx/include/module.modulemap (+3)
• (modified) libcxx/utils/generate_iwyu_mapping.py (+2)

diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 0f43916dae4384..a571832ab724d4 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -418,6 +418,7 @@ set(files
   __functional/weak_result_type.h
   __fwd/array.h
   __fwd/bit_reference.h
+  __fwd/byte.h
   __fwd/complex.h
   __fwd/deque.h
   __fwd/format.h
diff --git a/libcxx/include/__cstddef/byte.h b/libcxx/include/__cstddef/byte.h
index b8cfe5e8d1c7ef..09e1d75e0b41f6 100644
--- a/libcxx/include/__cstddef/byte.h
+++ b/libcxx/include/__cstddef/byte.h
@@ -10,6 +10,7 @@
 #define _LIBCPP___CSTDDEF_BYTE_H
 
 #include <__config>
+#include <__fwd/byte.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_integral.h>
 
diff --git a/libcxx/include/__fwd/byte.h b/libcxx/include/__fwd/byte.h
new file mode 100644
index 00000000000000..e15311961bebd4
--- /dev/null
+++ b/libcxx/include/__fwd/byte.h
@@ -0,0 +1,26 @@
+//===---------------------------------------------------------------------===//
+//
+// 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___FWD_BYTE_H
+#define _LIBCPP___FWD_BYTE_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 17
+namespace std { // purposefully not versioned
+
+enum class byte : unsigned char;
+
+}
+#endif // _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___FWD_BYTE_H
diff --git a/libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h b/libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h
index 337f878fea5c1d..15dda5824a3623 100644
--- a/libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h
+++ b/libcxx/include/__type_traits/is_trivially_lexicographically_comparable.h
@@ -10,13 +10,13 @@
 #define _LIBCPP___TYPE_TRAITS_IS_TRIVIALLY_LEXICOGRAPHICALLY_COMPARABLE_H
 
 #include <__config>
+#include <__fwd/byte.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_unsigned.h>
 #include <__type_traits/remove_cv.h>
 #include <__type_traits/void_t.h>
 #include <__utility/declval.h>
-#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 3abc11723a5a92..65df579b8d6dd7 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -636,6 +636,9 @@ module std_private_bit_reference     [system] {
 module std_private_fwd_bit_reference [system] {
   header "__fwd/bit_reference.h"
 }
+module std_private_fwd_byte [system] {
+  header "__fwd/byte.h"
+}
 module std_private_config            [system] {
   textual header "__config"
   textual header "__configuration/abi.h"
diff --git a/libcxx/utils/generate_iwyu_mapping.py b/libcxx/utils/generate_iwyu_mapping.py
index b0ebe1b9e93d26..599201808bb79b 100644
--- a/libcxx/utils/generate_iwyu_mapping.py
+++ b/libcxx/utils/generate_iwyu_mapping.py
@@ -40,6 +40,8 @@ def IWYU_mapping(header: str) -> typing.Optional[typing.List[str]]:
         return ["atomic", "mutex", "semaphore", "thread"]
     elif header == "__tree":
         return ["map", "set"]
+    elif header == "__fwd/byte.h":
+        return ["cstddef"]
     elif header == "__fwd/pair.h":
         return ["utility"]
     elif header == "__fwd/subrange.h":