Fix constant evaluation due to non-constant pointer_traits::pointer_to for gcc

Author: winner245

libcxx/include/forward_list

@@ -300,7 +300,16 @@ struct __forward_node_traits {
     return __p;
   }
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI static __begin_node_pointer __as_iter_node(__node_pointer __p) {
-    return __p ? pointer_traits<__begin_node_pointer>::pointer_to(*__p) : static_cast<__begin_node_pointer>(nullptr);
+#  ifdef _LIBCPP_CXX03_LANG
+    return static_cast<__begin_node_pointer>(__p);
+#  else
+    if constexpr (std::is_pointer<__begin_node_pointer>::value) {
+      return static_cast<__begin_node_pointer>(__p);
+    } else {
+      return __p ? __begin_node_pointer::pointer_to(*static_cast<__begin_node*>(std::addressof(*__p)))
+                 : static_cast<__begin_node_pointer>(nullptr);
+    }
+#  endif
   }
 };
 
@@ -315,8 +324,16 @@ struct __forward_begin_node {
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __forward_begin_node(pointer __n) : __next_(__n) {}
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __next_as_begin() const {
-    return __next_ ? pointer_traits<__begin_node_pointer>::pointer_to(*__next_)
-                   : static_cast<__begin_node_pointer>(nullptr);
+#  ifdef _LIBCPP_CXX03_LANG
+    return static_cast<__begin_node_pointer>(__next_);
+#  else
+    if constexpr (std::is_pointer<__begin_node_pointer>::value) {
+      return static_cast<__begin_node_pointer>(__next_);
+    } else {
+      return __next_ ? __begin_node_pointer::pointer_to(*static_cast<__forward_begin_node*>(std::addressof(*__next_)))
+                     : static_cast<__begin_node_pointer>(nullptr);
+    }
+#  endif
   }
 };
 
@@ -363,6 +380,7 @@ template <class _NodePtr>
 class _LIBCPP_TEMPLATE_VIS __forward_list_iterator {
   typedef __forward_node_traits<_NodePtr> __traits;
   typedef typename __traits::__node_type __node_type;
+  typedef typename __traits::__begin_node __begin_node_type;
   typedef typename __traits::__node_pointer __node_pointer;
   typedef typename __traits::__begin_node_pointer __begin_node_pointer;
   typedef typename __traits::__void_pointer __void_pointer;
@@ -371,9 +389,17 @@ class _LIBCPP_TEMPLATE_VIS __forward_list_iterator {
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const { return __ptr_; }
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __node_pointer __get_unsafe_node_pointer() const {
-    return __ptr_ ? pointer_traits<__node_pointer>::pointer_to(
-                        const_cast<__node_type&>(static_cast<const __node_type&>(*__ptr_)))
-                  : static_cast<__node_pointer>(nullptr);
+#  ifdef _LIBCPP_CXX03_LANG
+    return static_cast<__node_pointer>(__ptr_);
+#  else
+    if constexpr (std::is_pointer<__node_pointer>::value) {
+      return static_cast<__node_pointer>(__ptr_);
+    } else {
+      return __ptr_ ? __node_pointer::pointer_to(
+                          *static_cast<__node_type*>(const_cast<__begin_node_type*>(std::addressof(*__ptr_))))
+                    : static_cast<__node_pointer>(nullptr);
+    }
+#  endif
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __forward_list_iterator(nullptr_t) _NOEXCEPT
@@ -434,6 +460,7 @@ class _LIBCPP_TEMPLATE_VIS __forward_list_const_iterator {
 
   typedef __forward_node_traits<_NodePtr> __traits;
   typedef typename __traits::__node_type __node_type;
+  typedef typename __traits::__begin_node __begin_node_type;
   typedef typename __traits::__node_pointer __node_pointer;
   typedef typename __traits::__begin_node_pointer __begin_node_pointer;
   typedef typename __traits::__void_pointer __void_pointer;
@@ -442,9 +469,17 @@ class _LIBCPP_TEMPLATE_VIS __forward_list_const_iterator {
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __get_begin() const { return __ptr_; }
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __node_pointer __get_unsafe_node_pointer() const {
-    return __ptr_ ? pointer_traits<__node_pointer>::pointer_to(
-                        const_cast<__node_type&>(static_cast<const __node_type&>(*__ptr_)))
-                  : static_cast<__node_pointer>(nullptr);
+#  ifdef _LIBCPP_CXX03_LANG
+    return static_cast<__node_pointer>(__ptr_);
+#  else
+    if constexpr (std::is_pointer<__node_pointer>::value) {
+      return static_cast<__node_pointer>(__ptr_);
+    } else {
+      return __ptr_ ? __node_pointer::pointer_to(
+                          *static_cast<__node_type*>(const_cast<__begin_node_type*>(std::addressof(*__ptr_))))
+                    : static_cast<__node_pointer>(nullptr);
+    }
+#  endif
   }
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __forward_list_const_iterator(nullptr_t) _NOEXCEPT
@@ -518,10 +553,27 @@ protected:
   _LIBCPP_COMPRESSED_PAIR(__begin_node, __before_begin_, __node_allocator, __alloc_);
 
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __before_begin() _NOEXCEPT {
+#  ifdef _LIBCPP_CXX03_LANG
     return pointer_traits<__begin_node_pointer>::pointer_to(__before_begin_);
+#  else
+    if constexpr (std::is_pointer<__begin_node_pointer>::value) {
+      return std::addressof(__before_begin_);
+    } else {
+      return __begin_node_pointer::pointer_to(*std::addressof(__before_begin_));
+    }
+#  endif
   }
+
   _LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __begin_node_pointer __before_begin() const _NOEXCEPT {
+#  ifdef _LIBCPP_CXX03_LANG
     return pointer_traits<__begin_node_pointer>::pointer_to(const_cast<__begin_node&>(__before_begin_));
+#  else
+    if constexpr (std::is_pointer<__begin_node_pointer>::value) {
+      return const_cast<__begin_node*>(std::addressof(__before_begin_));
+    } else {
+      return __begin_node_pointer::pointer_to(*const_cast<__begin_node*>(std::addressof(__before_begin_)));
+    }
+#  endif
   }
 
   typedef __forward_list_iterator<__node_pointer> iterator;