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

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merged 2 commits into from
Sep 14, 2023
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[libc][utils] cpp::always_false to enable static_assert(false) #66209

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1e6278e
[libc][utils] cpp::always_false to enable static_assert(false)
gchatelet Sep 13, 2023
e5ab8fd
Add more documentation
gchatelet Sep 13, 2023
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1 change: 1 addition & 0 deletions libc/src/__support/CPP/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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29 changes: 29 additions & 0 deletions libc/src/__support/CPP/type_traits/always_false.h
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@@ -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
8 changes: 2 additions & 6 deletions libc/src/__support/CPP/utility/declval.h
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Expand Up @@ -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");
}

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1 change: 1 addition & 0 deletions utils/bazel/llvm-project-overlay/libc/BUILD.bazel
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Expand Up @@ -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",
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