libstdc++: Improve std::forward static assert message

The previous message told you something was wrong, but not why it
happened or why it's bad. This changes it to explain that the function
is being misused.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/move.h (forward(remove_reference_t<T>&&)):
	Improve text of static_assert.
	* testsuite/20_util/forward/c_neg.cc: Adjust dg-error.
	* testsuite/20_util/forward/f_neg.cc: Likewise.

(cherry picked from commit a11052d98db2f2a61841f0c5ee84de4ca1b3e296)

diff --git a/libstdc++-v3/include/bits/move.h b/libstdc++-v3/include/bits/move.h
index 5a4dbdc823c7f945c175118cc57fdc0042584b39..b5a4020eeee6c065407064804ce885baf3edfb37 100644 (file)
--- a/libstdc++-v3/include/bits/move.h
+++ b/libstdc++-v3/include/bits/move.h
@@ -86,8 +86,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr _Tp&&
     forward(typename std::remove_reference<_Tp>::type&& __t) noexcept
     {
-      static_assert(!std::is_lvalue_reference<_Tp>::value, "template argument"
-                   " substituting _Tp is an lvalue reference type");
+      static_assert(!std::is_lvalue_reference<_Tp>::value,
+         "std::forward must not be used to convert an rvalue to an lvalue");
       return static_cast<_Tp&&>(__t);
     }
 

diff --git a/libstdc++-v3/testsuite/20_util/forward/c_neg.cc b/libstdc++-v3/testsuite/20_util/forward/c_neg.cc
index 8b6736d42f0c16173e6d12be0087a0e702024820..5e318ed67f44ea7529430c117de7c1dc58985997 100644 (file)
--- a/libstdc++-v3/testsuite/20_util/forward/c_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/forward/c_neg.cc
@@ -17,7 +17,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// { dg-error "static assertion failed" "" { target *-*-* } 89 }
+// { dg-error "convert an rvalue to an lvalue" "" { target *-*-* } 0 }
 
 #include <list>
 

diff --git a/libstdc++-v3/testsuite/20_util/forward/f_neg.cc b/libstdc++-v3/testsuite/20_util/forward/f_neg.cc
index 4b697f1cd5d83fcad74121c73d5a74d56754923e..c6cf880bd15142e46b762eac256fd2fa20958e5d 100644 (file)
--- a/libstdc++-v3/testsuite/20_util/forward/f_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/forward/f_neg.cc
@@ -17,7 +17,7 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// { dg-error "static assertion failed" "" { target *-*-* } 89 }
+// { dg-error "convert an rvalue to an lvalue" "" { target *-*-* } 0 }
 
 #include <utility>