https://gcc.gnu.org/g:140aab25a4865433d987d73c57f4ddf11fdac1e2
commit r14-10654-g140aab25a4865433d987d73c57f4ddf11fdac1e2
Author: Patrick Palka <ppa...@redhat.com>
Date: Sat Aug 3 09:05:05 2024 -0400
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 <jwak...@redhat.com>
(cherry picked from commit 8256d5c0097dff00f9bdf9ee0c9d53bd7cec2802)
Diff:
---
libstdc++-v3/include/bits/move.h | 47 +++++++++++-----------
.../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 bb200c95964a..8397a01b6323 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 ff835af19151..5dafa419a7ea 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
