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| author | Patrick Palka <ppalka@redhat.com> | 2024-08-03 09:05:05 -0400 |
|---|---|---|
| committer | Patrick Palka <ppalka@redhat.com> | 2024-08-03 09:05:05 -0400 |
| commit | 8256d5c0097dff00f9bdf9ee0c9d53bd7cec2802 (patch) | |
| tree | d59e68a9a9566777a5c4295ece90d7c1c753b69d | |
| parent | db9834aead7629c3f8d640fa546fdc84780c354a (diff) | |
| download | gcc-8256d5c0097dff00f9bdf9ee0c9d53bd7cec2802.zip gcc-8256d5c0097dff00f9bdf9ee0c9d53bd7cec2802.tar.gz gcc-8256d5c0097dff00f9bdf9ee0c9d53bd7cec2802.tar.bz2 | |
libstdc++: use concrete return type for std::forward_like
Inspired by https://github.com/llvm/llvm-project/issues/101614 this
inverts the relationship between forward_like and __like_t so that
forward_like is defined in terms of __like_t and with a concrete return
type. __like_t in turn is defined via partial specializations that
pattern match on the const- and reference-ness of T.
This turns out to be more SFINAE friendly and significantly cheaper
to compile than the previous implementation.
libstdc++-v3/ChangeLog:
* include/bits/move.h (__like_impl): New metafunction.
(__like_t): Redefine in terms of __like_impl.
(forward_like): Redefine in terms of __like_t.
* testsuite/20_util/forward_like/2_neg.cc: Don't expect
error outside the immediate context anymore.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
| -rw-r--r-- | libstdc++-v3/include/bits/move.h | 47 | ||||
| -rw-r--r-- | libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc | 6 |
2 files changed, 26 insertions, 27 deletions
diff --git a/libstdc++-v3/include/bits/move.h b/libstdc++-v3/include/bits/move.h index bb200c9..8397a01 100644 --- a/libstdc++-v3/include/bits/move.h +++ b/libstdc++-v3/include/bits/move.h @@ -88,31 +88,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION #if __glibcxx_forward_like // C++ >= 23 template<typename _Tp, typename _Up> - [[nodiscard]] - constexpr decltype(auto) - forward_like(_Up&& __x) noexcept - { - constexpr bool __as_rval = is_rvalue_reference_v<_Tp&&>; - - if constexpr (is_const_v<remove_reference_t<_Tp>>) - { - using _Up2 = remove_reference_t<_Up>; - if constexpr (__as_rval) - return static_cast<const _Up2&&>(__x); - else - return static_cast<const _Up2&>(__x); - } - else - { - if constexpr (__as_rval) - return static_cast<remove_reference_t<_Up>&&>(__x); - else - return static_cast<_Up&>(__x); - } - } + struct __like_impl; // _Tp must be a reference and _Up an lvalue reference + + template<typename _Tp, typename _Up> + struct __like_impl<_Tp&, _Up&> + { using type = _Up&; }; + + template<typename _Tp, typename _Up> + struct __like_impl<const _Tp&, _Up&> + { using type = const _Up&; }; + + template<typename _Tp, typename _Up> + struct __like_impl<_Tp&&, _Up&> + { using type = _Up&&; }; + + template<typename _Tp, typename _Up> + struct __like_impl<const _Tp&&, _Up&> + { using type = const _Up&&; }; template<typename _Tp, typename _Up> - using __like_t = decltype(std::forward_like<_Tp>(std::declval<_Up>())); + using __like_t = typename __like_impl<_Tp&&, _Up&>::type; + + template<typename _Tp, typename _Up> + [[nodiscard]] + constexpr __like_t<_Tp, _Up> + forward_like(_Up&& __x) noexcept + { return static_cast<__like_t<_Tp, _Up>>(__x); } #endif /** diff --git a/libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc b/libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc index ff835af..5dafa41 100644 --- a/libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc +++ b/libstdc++-v3/testsuite/20_util/forward_like/2_neg.cc @@ -2,9 +2,7 @@ #include <utility> -auto x1 = std::forward_like<void>(1); // { dg-error "here" } +auto x1 = std::forward_like<void>(1); // { dg-error "no match" } // { dg-error "forming reference to void" "" { target *-*-* } 0 } -auto x2 = std::forward_like<void()const>(1); // { dg-error "here" } +auto x2 = std::forward_like<void()const>(1); // { dg-error "no match" } // { dg-error "forming reference to qualified function" "" { target *-*-* } 0 } - -// { dg-prune-output "inconsistent deduction for auto return type" } // PR111484 |