| // Copyright 2022 The Chromium Authors |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef BASE_TYPES_ALWAYS_FALSE_H_ |
| #define BASE_TYPES_ALWAYS_FALSE_H_ |
| |
| namespace base { |
| |
| // A helper that can be used with a static_assert() that must always fail (e.g. |
| // for an undesirable template instantiation). Such a static_assert() cannot |
| // simply be written as static_assert(false, ...) because that would always fail |
| // to compile, even if the template was never instantiated. Instead, a common |
| // idiom is to force the static_assert() to depend on a template parameter so |
| // that it is only evaluated when the template is instantiated: |
| // |
| // template <typename U = T> |
| // void SomeDangerousMethodThatShouldNeverCompile() { |
| // static_assert(base::AlwaysFalse<U>, "explanatory message here"); |
| // } |
| // |
| // |
| // The issue of not being able to use static_assert(false, ...) in a |
| // non-instantiated template was fixed in C++23. When Chromium switches to |
| // building with C++23, remove this file and use false directly, and search |
| // across the Chromium codebase for "AlwaysFalse", as there are other |
| // implementations in places that cannot depend on this file. |
| // |
| // References: |
| // - https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2593r0.html |
| // - https://github.com/cplusplus/papers/issues/1251 |
| |
| namespace internal { |
| |
| template <typename... Args> |
| struct AlwaysFalseHelper { |
| static constexpr bool kValue = false; |
| }; |
| |
| } // namespace internal |
| |
| template <typename... Args> |
| inline constexpr bool AlwaysFalse = |
| internal::AlwaysFalseHelper<Args...>::kValue; |
| |
| } // namespace base |
| |
| #endif // BASE_TYPES_ALWAYS_FALSE_H_ |
