[libcxx-commits] [libcxx] [libcxx] improves diagnostics for containers with bad value types (PR #106296)

[Louis Dionne via libcxx-commits]

================
@@ -635,8 +640,136 @@ void __forward_list_base<_Tp, _Alloc>::clear() _NOEXCEPT {
   __before_begin()->__next_ = nullptr;
 }
 
+// begin-diagnostic-helpers
+// std::forward_list can only have non-cv-qualified object types as its value type, which we diagnose.
+// In order to short-cirucit redundant (and cryptic) diagnostics, std::forward_list must be the one
+// to fire the static_assert. Since std::forward_list inherits from __forward_list_base, we also need
+// to create specialisations for the rejected types, so that everything appears "good" to std::forward_list
+// (otherwise we'll get lots of unhelpful diagnostics that suppress the one std::forward_list offers).
+template <class _Tp, class _Alloc>
+class _LIBCPP_TEMPLATE_VIS __forward_list_base<_Tp const, _Alloc> {
+public:
+  using __node_allocator = void*;
+  using __node_alloc_traits = void*;
+  using __node_pointer = void*;
+  using __node_type = void*;
+  using __node_base = void*;
+  using __node_base_pointer = void*;
+  using __link_pointer = void*;
+
+  using pointer = int*;
+  using const_pointer = int const*;
+  using size_type = unsigned int;
+  using difference_type = int;
+  using iterator = int*;
+  using const_iterator = int const*;
+};
+
+template <class _Tp, class _Alloc>
+class _LIBCPP_TEMPLATE_VIS __forward_list_base<_Tp volatile, _Alloc> {
+public:
+  using __node_allocator = void*;
+  using __node_alloc_traits = void*;
+  using __node_pointer = void*;
+  using __node_type = void*;
+  using __node_base = void*;
+  using __node_base_pointer = void*;
+  using __link_pointer = void*;
+
+  using pointer = int*;
+  using const_pointer = int const*;
+  using size_type = unsigned int;
+  using difference_type = int;
+  using iterator = int*;
+  using const_iterator = int const*;
+};
+
+template <class _Tp, class _Alloc>
+class _LIBCPP_TEMPLATE_VIS __forward_list_base<_Tp&, _Alloc> {
+public:
+  using __node_allocator = void*;
+  using __node_alloc_traits = void*;
+  using __node_pointer = void*;
+  using __node_type = void*;
+  using __node_base = void*;
+  using __node_base_pointer = void*;
+  using __link_pointer = void*;
+
+  using pointer = int*;
+  using const_pointer = int const*;
+  using size_type = unsigned int;
+  using difference_type = int;
+  using iterator = int*;
+  using const_iterator = int const*;
+};
+
+template <class _Tp, class _Alloc>
+class _LIBCPP_TEMPLATE_VIS __forward_list_base<_Tp&&, _Alloc> {
+public:
+  using __node_allocator = void*;
+  using __node_alloc_traits = void*;
+  using __node_pointer = void*;
+  using __node_type = void*;
+  using __node_base = void*;
+  using __node_base_pointer = void*;
+  using __link_pointer = void*;
+
+  using pointer = int*;
+  using const_pointer = int const*;
+  using size_type = unsigned int;
+  using difference_type = int;
+  using iterator = int*;
+  using const_iterator = int const*;
+};
+
+template <class _Tp, class... _Args, class _Alloc>
+class _LIBCPP_TEMPLATE_VIS __forward_list_base<_Tp(_Args...), _Alloc> {
+public:
+  using __node_allocator = void*;
+  using __node_alloc_traits = void*;
+  using __node_pointer = void*;
+  using __node_type = void*;
+  using __node_base = void*;
+  using __node_base_pointer = void*;
+  using __link_pointer = void*;
+
+  using pointer = int*;
+  using const_pointer = int const*;
+  using size_type = unsigned int;
+  using difference_type = int;
+  using iterator = int*;
+  using const_iterator = int const*;
+};
+
+template <class _Alloc>
+class _LIBCPP_TEMPLATE_VIS __forward_list_base<void, _Alloc> {
+public:
+  using __node_allocator = void*;
+  using __node_alloc_traits = void*;
+  using __node_pointer = void*;
+  using __node_type = void*;
+  using __node_base = void*;
+  using __node_base_pointer = void*;
+  using __link_pointer = void*;
+
+  using pointer = int*;
+  using const_pointer = int const*;
+  using size_type = unsigned int;
+  using difference_type = int;
+  using iterator = int*;
+  using const_iterator = int const*;
+};
+// end-diagnostic-helpers
+
 template <class _Tp, class _Alloc /*= allocator<_Tp>*/>
 class _LIBCPP_TEMPLATE_VIS forward_list : private __forward_list_base<_Tp, _Alloc> {
+  static_assert(!is_const<_Tp>::value, "'std::forward_list' can only hold non-const types");
----------------
ldionne wrote:

> Related, but any resolution is beyond the scope of this patch: is __forward_list_base actually necessary? I've only skimmed it, but can't see anything that would require forward_list to need a base class (similarly for std::list's base).

It might not be necessary, but one reason I've seen these base classes used for is exception safety. `forward_list` constructs an instance of the base class and then does something else, and if something goes wrong in there, the base class will be properly destroyed. IDK if that's the case here but that's the first thing I'd look out for if I was trying to refactor this.

https://github.com/llvm/llvm-project/pull/106296