[libc][utils] cpp::always_false to enable static_assert(false) (#66209)

llvm · Sep 14, 2023 · aee8f87 · aee8f87
1 parent 84216d1
commit aee8f87
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Showing 4 changed files with 33 additions and 6 deletions.
diff --git a/libc/src/__support/CPP/CMakeLists.txt b/libc/src/__support/CPP/CMakeLists.txt
@@ -95,6 +95,7 @@ add_header_library(
   type_traits
   HDRS
     type_traits.h
+    type_traits/always_false.h
     type_traits/add_lvalue_reference.h
     type_traits/add_pointer.h
     type_traits/add_rvalue_reference.h

diff --git a/libc/src/__support/CPP/type_traits/always_false.h b/libc/src/__support/CPP/type_traits/always_false.h
@@ -0,0 +1,29 @@
+//===-- convenient static_assert(false) helper ------------------*- 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 LLVM_LIBC_SRC_SUPPORT_CPP_TYPE_TRAITS_ALWAYS_FALSE_H
+#define LLVM_LIBC_SRC_SUPPORT_CPP_TYPE_TRAITS_ALWAYS_FALSE_H
+
+#include "src/__support/macros/attributes.h"
+
+namespace __llvm_libc::cpp {
+
+// This is technically not part of the standard but it come often enough that
+// it's convenient to have around.
+//
+// https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2593r0.html#valid-workaround
+//
+// This will be fixed in C++23 according to [CWG
+// 2518](https://cplusplus.github.io/CWG/issues/2518.html).
+
+// Usage `static_assert(cpp::always_false<T>, "error message");`
+template <typename...> LIBC_INLINE_VAR constexpr bool always_false = false;
+
+} // namespace __llvm_libc::cpp
+
+#endif // LLVM_LIBC_SRC_SUPPORT_CPP_TYPE_TRAITS_ALWAYS_FALSE_H
diff --git a/libc/src/__support/CPP/utility/declval.h b/libc/src/__support/CPP/utility/declval.h
@@ -9,17 +9,13 @@
 #define LLVM_LIBC_SRC_SUPPORT_CPP_UTILITY_DECLVAL_H
 
 #include "src/__support/CPP/type_traits/add_rvalue_reference.h"
-#include "src/__support/macros/attributes.h"
+#include "src/__support/CPP/type_traits/always_false.h"
 
 namespace __llvm_libc::cpp {
 
 // declval
-namespace detail {
-template <typename T> LIBC_INLINE_VAR constexpr bool always_false = false;
-}
-
 template <typename T> cpp::add_rvalue_reference_t<T> declval() {
-  static_assert(detail::always_false<T>,
+  static_assert(cpp::always_false<T>,
                 "declval not allowed in an evaluated context");
 }
 

diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -289,6 +289,7 @@ libc_support_library(
         "src/__support/CPP/type_traits/add_lvalue_reference.h",
         "src/__support/CPP/type_traits/add_pointer.h",
         "src/__support/CPP/type_traits/add_rvalue_reference.h",
+        "src/__support/CPP/type_traits/always_false.h",
         "src/__support/CPP/type_traits/bool_constant.h",
         "src/__support/CPP/type_traits/conditional.h",
         "src/__support/CPP/type_traits/decay.h",