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| 1 | //===----------------------------------------------------------------------===// |
|---|---|
| 2 | // |
| 3 | // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| 4 | // See https://llvm.org/LICENSE.txt for license information. |
| 5 | // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| 6 | // |
| 7 | //===----------------------------------------------------------------------===// |
| 8 | |
| 9 | #ifndef _LIBCPP___TYPE_TRAITS_DESUGARS_TO_H |
| 10 | #define _LIBCPP___TYPE_TRAITS_DESUGARS_TO_H |
| 11 | |
| 12 | #include <__config> |
| 13 | |
| 14 | #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) |
| 15 | # pragma GCC system_header |
| 16 | #endif |
| 17 | |
| 18 | _LIBCPP_BEGIN_NAMESPACE_STD |
| 19 | |
| 20 | // Tags to represent the canonical operations. |
| 21 | |
| 22 | // syntactically, the operation is equivalent to calling `a == b` |
| 23 | struct __equal_tag {}; |
| 24 | |
| 25 | // syntactically, the operation is equivalent to calling `a + b` |
| 26 | struct __plus_tag {}; |
| 27 | |
| 28 | // syntactically, the operation is equivalent to calling `a < b` |
| 29 | struct __less_tag {}; |
| 30 | |
| 31 | // syntactically, the operation is equivalent to calling `a > b` |
| 32 | struct __greater_tag {}; |
| 33 | |
| 34 | // syntactically, the operation is equivalent to calling `a < b`, and these expressions |
| 35 | // have to be true for any `a` and `b`: |
| 36 | // - `(a < b) == (b > a)` |
| 37 | // - `(!(a < b) && !(b < a)) == (a == b)` |
| 38 | // For example, this is satisfied for std::less on integral types, but also for ranges::less on all types due to |
| 39 | // additional semantic requirements on that operation. |
| 40 | struct __totally_ordered_less_tag {}; |
| 41 | |
| 42 | // This class template is used to determine whether an operation "desugars" |
| 43 | // (or boils down) to a given canonical operation. |
| 44 | // |
| 45 | // For example, `std::equal_to<>`, our internal `std::__equal_to` helper and |
| 46 | // `ranges::equal_to` are all just fancy ways of representing a transparent |
| 47 | // equality operation, so they all desugar to `__equal_tag`. |
| 48 | // |
| 49 | // This is useful to optimize some functions in cases where we know e.g. the |
| 50 | // predicate being passed is actually going to call a builtin operator, or has |
| 51 | // some specific semantics. |
| 52 | template <class _CanonicalTag, class _Operation, class... _Args> |
| 53 | inline const bool __desugars_to_v = false; |
| 54 | |
| 55 | // For the purpose of determining whether something desugars to something else, |
| 56 | // we disregard const and ref qualifiers on the operation itself. |
| 57 | template <class _CanonicalTag, class _Operation, class... _Args> |
| 58 | inline const bool __desugars_to_v<_CanonicalTag, _Operation const, _Args...> = |
| 59 | __desugars_to_v<_CanonicalTag, _Operation, _Args...>; |
| 60 | template <class _CanonicalTag, class _Operation, class... _Args> |
| 61 | inline const bool __desugars_to_v<_CanonicalTag, _Operation&, _Args...> = |
| 62 | __desugars_to_v<_CanonicalTag, _Operation, _Args...>; |
| 63 | template <class _CanonicalTag, class _Operation, class... _Args> |
| 64 | inline const bool __desugars_to_v<_CanonicalTag, _Operation&&, _Args...> = |
| 65 | __desugars_to_v<_CanonicalTag, _Operation, _Args...>; |
| 66 | |
| 67 | _LIBCPP_END_NAMESPACE_STD |
| 68 | |
| 69 | #endif // _LIBCPP___TYPE_TRAITS_DESUGARS_TO_H |
| 70 |
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