libstdc++: Simplify std::list assignment using 'if constexpr'

Use diagnostic pragmas to allow using `if constexpr` in C++11 mode, so
that we don't need to use tag dispatching.

The _M_move_assign overloads that were previously used for tag
dispatching are no longer used, but are retained here (at least for the
default config) so that an explicit instantiation will still define
those members. This ensures that old code which expects an explicit
instantiation in some other translation unit will still link. I'm not
sure if that's really needed, we should probably have a policy about
whether we support explicit instantiations where the declaration and
definition use different versions of the headers.

libstdc++-v3/ChangeLog:

	* include/bits/stl_list.h (operator=(list&&)): Use if constexpr
	instead of dispatching to _M_move_assign.
	(_M_move_assign): Do not define for versioned namespace.

diff --git a/libstdc++-v3/include/bits/stl_list.h b/libstdc++-v3/include/bits/stl_list.h
index 7deb04b4bfe42aa53162d14b981cd77be29de19c..3f92de42996e3b55581daa2a73ae221b6f0de407 100644 (file)
--- a/libstdc++-v3/include/bits/stl_list.h
+++ b/libstdc++-v3/include/bits/stl_list.h
@@ -935,6 +935,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       operator=(const list& __x);
 
 #if __cplusplus >= 201103L
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
       /**
        *  @brief  %List move assignment operator.
        *  @param  __x  A %list of identical element and allocator types.
@@ -952,9 +954,29 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
        constexpr bool __move_storage =
          _Node_alloc_traits::_S_propagate_on_move_assign()
          || _Node_alloc_traits::_S_always_equal();
-       _M_move_assign(std::move(__x), __bool_constant<__move_storage>());
+       if constexpr (!__move_storage)
+         {
+           if (__x._M_get_Node_allocator() != this->_M_get_Node_allocator())
+             {
+               // The rvalue's allocator cannot be moved, or is not equal,
+               // so we need to individually move each element.
+               _M_assign_dispatch(std::make_move_iterator(__x.begin()),
+                                  std::make_move_iterator(__x.end()),
+                                  __false_type{});
+               return *this;
+             }
+         }
+
+       this->clear();
+       this->_M_move_nodes(std::move(__x));
+
+       if constexpr (_Node_alloc_traits::_S_propagate_on_move_assign())
+         this->_M_get_Node_allocator()
+             = std::move(__x._M_get_Node_allocator());
+
        return *this;
       }
+#pragma GCC diagnostic pop
 
       /**
        *  @brief  %List initializer list assignment operator.
@@ -2156,7 +2178,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       const_iterator
       _M_resize_pos(size_type& __new_size) const;
 
-#if __cplusplus >= 201103L
+#if __cplusplus >= 201103L && ! _GLIBCXX_INLINE_VERSION
+      // XXX GLIBCXX_ABI Deprecated
+      // These are unused and only kept so that explicit instantiations will
+      // continue to define the symbols.
       void
       _M_move_assign(list&& __x, true_type) noexcept
       {