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.

diff --git a/libstdc++-v3/include/bits/move.h b/libstdc++-v3/include/bits/move.h
index 3abbb37..2dd7ed9 100644 (file)
--- a/libstdc++-v3/include/bits/move.h
+++ b/libstdc++-v3/include/bits/move.h
@@ -88,8 +88,8 @@ _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 must not be 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 dc7ec51..3875792 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 "must not be an lvalue reference" "" { target *-*-* } 0 }
+// { 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 4ccd726..51ccaf2 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 "must not be an lvalue reference" "" { target *-*-* } 0 }
+// { dg-error "convert an rvalue to an lvalue" "" { target *-*-* } 0 }
 
 #include <utility>