[committed] libstdc++: Improve std::forward static assert message

[committed] libstdc++: Improve std::forward static assert message

[Jonathan Wakely]

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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.

Tested x86_64-linux. Committed to trunk.


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commit a11052d98db2f2a61841f0c5ee84de4ca1b3e296
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 28 12:35:29 2021

    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 3abbb37ceeb..2dd7ed9e4f9 100644
--- 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 dc7ec51bde6..3875792866e 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 "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 4ccd7264c65..51ccaf29c1a 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 "must not be an lvalue reference" "" { target *-*-* } 0 }
+// { dg-error "convert an rvalue to an lvalue" "" { target *-*-* } 0 }
 
 #include <utility>