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>

diff --git a/libstdc++-v3/include/bits/move.h b/libstdc++-v3/include/bits/move.h
index bb200c95964aa849981c1c6ee4943d47f49baed6..8397a01b6323a53ae29787af9fe854581ea100f9 100644 (file)
--- 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 ff835af191511cf0de00ca02dc951e9d4a95e5cb..5dafa419a7ea8bfb913ff119683cc7c4c4078d7e 100644 (file)
--- 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