[committed] libstdc++: Fix std::forward_list::assign assignable check [PR122661]

[Jonathan Wakely]

The std::is_assignable check should test for assignment to an lvalue,
not an rvalue.

libstdc++-v3/ChangeLog:

        PR libstdc++/122661
        * include/bits/forward_list.h (forward_list::assign(I, I)): Fix
        value category in is_assignable check.
        * testsuite/23_containers/forward_list/modifiers/122661.cc:
        New test.
---

Tested powerpc64le-linux.

Pushed to trunk. This should be backported to all active branches too.

 libstdc++-v3/include/bits/forward_list.h      |  2 +-
 .../forward_list/modifiers/122661.cc          | 20 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 
libstdc++-v3/testsuite/23_containers/forward_list/modifiers/122661.cc

diff --git a/libstdc++-v3/include/bits/forward_list.h 
b/libstdc++-v3/include/bits/forward_list.h
index 8bcfb809319e..459b2f65ad18 100644
--- a/libstdc++-v3/include/bits/forward_list.h
+++ b/libstdc++-v3/include/bits/forward_list.h
@@ -1046,7 +1046,7 @@ namespace __fwdlist
        void
        assign(_InputIterator __first, _InputIterator __last)
        {
-         if constexpr (is_assignable<_Tp, decltype(*__first)>::value)
+         if constexpr (is_assignable<_Tp&, decltype(*__first)>::value)
            {
              auto __prev = before_begin();
              auto __curr = begin();
diff --git 
a/libstdc++-v3/testsuite/23_containers/forward_list/modifiers/122661.cc 
b/libstdc++-v3/testsuite/23_containers/forward_list/modifiers/122661.cc
new file mode 100644
index 000000000000..41ac32c7ec6a
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/forward_list/modifiers/122661.cc
@@ -0,0 +1,20 @@
+// { dg-do compile { target c++11 } }
+
+// Bug 122661 - Incorrect value category handling in forward_list::assign
+
+#include <forward_list>
+
+struct S
+{
+  S();
+  S& operator=(S const&) & = delete;
+  S& operator=(S const&) &&;
+};
+
+void
+test_pr122661()
+{
+  std::forward_list<S> fl;
+  S* iter = nullptr;
+  fl.assign(iter, iter);
+}
-- 
2.51.1