[gcc r10-10303] libstdc++: Improve std::forward static assert message

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:453a8f3d3966f024fae8b8bb40f968e4be2b458e

commit r10-10303-g453a8f3d3966f024fae8b8bb40f968e4be2b458e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 28 12:35:29 2021 +0100

    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:
---
 libstdc++-v3/include/bits/move.h                | 4 ++--
 libstdc++-v3/testsuite/20_util/forward/c_neg.cc | 2 +-
 libstdc++-v3/testsuite/20_util/forward/f_neg.cc | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/move.h b/libstdc++-v3/include/bits/move.h
index 5a4dbdc823c..b5a4020eeee 100644
--- 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 8b6736d42f0..5e318ed67f4 100644
--- 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 4b697f1cd5d..c6cf880bd15 100644
--- 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>