[libcxx] improves diagnostics for containers with bad value types

libcxx/include/CMakeLists.txt

@@ -754,6 +754,7 @@ set(files
   __type_traits/decay.h
   __type_traits/dependent_type.h
   __type_traits/desugars_to.h
+  __type_traits/diagnostic_utilities.h
   __type_traits/disjunction.h
   __type_traits/enable_if.h
   __type_traits/extent.h

libcxx/include/__memory/allocator.h

@@ -14,11 +14,10 @@
 #include <__memory/addressof.h>
 #include <__memory/allocate_at_least.h>
 #include <__memory/allocator_traits.h>
-#include <__type_traits/is_const.h>
+#include <__type_traits/diagnostic_utilities.h>
 #include <__type_traits/is_constant_evaluated.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_void.h>
-#include <__type_traits/is_volatile.h>
 #include <__utility/forward.h>
 #include <cstddef>
 #include <new>
@@ -76,8 +75,7 @@ struct __non_trivial_if<true, _Unique> {
 
 template <class _Tp>
 class _LIBCPP_TEMPLATE_VIS allocator : private __non_trivial_if<!is_void<_Tp>::value, allocator<_Tp> > {
-  static_assert(!is_const<_Tp>::value, "std::allocator does not support const types");
-  static_assert(!is_volatile<_Tp>::value, "std::allocator does not support volatile types");
+  static_assert(__allocator_requirements<allocator, _Tp>::value);
 
 public:
   typedef size_t size_type;
@@ -170,6 +168,8 @@ inline _LIBCPP_HIDE_FROM_ABI bool operator!=(const allocator<_Tp>&, const alloca
 
 #endif
 
+_LIBCPP_DEFINE__ALLOCATOR_VALUE_TYPE_REQUIREMENTS(allocator, "allocate");
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___MEMORY_ALLOCATOR_H

libcxx/include/__type_traits/diagnostic_utilities.h

@@ -0,0 +1,94 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_DIAGNOSTIC_UTILITIES_H
+#define _LIBCPP___TYPE_TRAITS_DIAGNOSTIC_UTILITIES_H
+
+#include <__config>
+#include <__type_traits/decay.h>
+#include <__type_traits/integral_constant.h>
+#include <__type_traits/is_bounded_array.h>
+#include <__type_traits/is_const.h>
+#include <__type_traits/is_function.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_unbounded_array.h>
+#include <__type_traits/is_void.h>
+#include <__type_traits/is_volatile.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// Many templates require their type parameters to be cv-unqualified objects.
+template <template <class...> class _Template, class _Tp, bool = is_same<__decay_t<_Tp>, _Tp>::value>
+struct __requires_cv_unqualified_object_type : true_type {};
+
+#define _LIBCPP_DEFINE__REQUIRES_CV_UNQUALIFIED_OBJECT_TYPE(_Template, _Verb)                                          \
+  template <class _Tp>                                                                                                 \
+  struct __requires_cv_unqualified_object_type<_Template, _Tp, false>                                                  \
+      : integral_constant<bool,                                                                                        \
+                          !(is_const<_Tp>::value || is_volatile<_Tp>::value || is_reference<_Tp>::value ||             \
+                            is_function<_Tp>::value)> {                                                                \
+    static_assert(!is_const<_Tp>::value, "'std::" #_Template "' cannot " _Verb " const types");                        \
+    static_assert(!is_volatile<_Tp>::value, "'std::" #_Template "' cannot " _Verb " volatile types");                  \
+    static_assert(!is_reference<_Tp>::value, "'std::" #_Template "' cannot " _Verb " references");                     \
+    static_assert(!is_function<_Tp>::value, "'std::" #_Template "' cannot " _Verb " functions");                       \
+  }
+
+// Per https://eel.is/c++draft/containers#container.reqmts-64, allocator-aware containers must have an
+// allocator that meets the Cpp17Allocator requirements (https://eel.is/c++draft/allocator.requirements).
+// In particular, this means that containers should only accept non-cv-qualified object types, and
+// types that are Cpp17Erasable.
+template <template <class...> class _Template, class _Tp, bool = is_same<__decay_t<_Tp>, _Tp>::value>
+struct __allocator_requirements : true_type {};
+
+#if _LIBCPP_STD_VER >= 20
+template <class _Tp>
+inline const bool __bounded_arrays_allowed_only_after_cxx20 = false;
+#else
+template <class _Tp>
+inline const bool __bounded_arrays_allowed_only_after_cxx20 = __libcpp_is_bounded_array<_Tp>::value;
+#endif
+
+#define _LIBCPP_DEFINE__ALLOCATOR_VALUE_TYPE_REQUIREMENTS(_Template, _Verb)                                            \
+  _LIBCPP_DEFINE__REQUIRES_CV_UNQUALIFIED_OBJECT_TYPE(_Template, _Verb);                                               \
+  template <class _Tp>                                                                                                 \
+  struct __allocator_requirements<_Template, _Tp, false>                                                               \
+      : integral_constant<bool,                                                                                        \
+                          __requires_cv_unqualified_object_type<_Template, _Tp>::value &&                              \
+                              !__bounded_arrays_allowed_only_after_cxx20<_Tp> > {                                      \
+    static_assert(!__bounded_arrays_allowed_only_after_cxx20<_Tp>,                                                     \
+                  "'std::" #_Template "' cannot " _Verb " C arrays before C++20");                                     \
+  }
+
+template <template <class...> class, class>
+struct __container_requirements : false_type {
+  static_assert(
+      false,
+      "a new container has been defined; please define '_LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS' for "
+      "that container");
+};
+
+#define _LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS(_Template)                                                   \
+  _LIBCPP_DEFINE__ALLOCATOR_VALUE_TYPE_REQUIREMENTS(_Template, "hold");                                                \
+  template <class _Tp>                                                                                                 \
+  struct __container_requirements<_Template, _Tp>                                                                      \
+      : integral_constant<bool,                                                                                        \
+                          __allocator_requirements<_Template, _Tp>::value &&                                           \
+                              !(is_void<_Tp>::value || __libcpp_is_unbounded_array<_Tp>::value)> {                     \
+    static_assert(!is_void<_Tp>::value, "'std::" #_Template "' cannot hold 'void'");                                   \
+    static_assert(!__libcpp_is_unbounded_array<_Tp>::value,                                                            \
+                  "'std::" #_Template "' cannot hold C arrays of an unknown size");                                    \
+  }
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_DIAGNOSTIC_UTILITIES_H

libcxx/include/array

@@ -128,10 +128,14 @@ template <size_t I, class T, size_t N> const T&& get(const array<T, N>&&) noexce
 #include <__type_traits/is_array.h>
 #include <__type_traits/is_const.h>
 #include <__type_traits/is_constructible.h>
+#include <__type_traits/is_function.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/is_reference.h>
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_relocatable.h>
+#include <__type_traits/is_unbounded_array.h>
+#include <__type_traits/is_void.h>
 #include <__type_traits/remove_cv.h>
 #include <__utility/empty.h>
 #include <__utility/integer_sequence.h>
@@ -168,6 +172,11 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp, size_t _Size>
 struct _LIBCPP_TEMPLATE_VIS array {
+  static_assert(!is_reference<_Tp>::value, "'std::array' cannot hold references");
+  static_assert(!is_function<_Tp>::value, "'std::array' cannot hold functions");
+  static_assert(!is_void<_Tp>::value, "'std::array' cannot hold 'void'");
+  static_assert(!__libcpp_is_unbounded_array<_Tp>::value, "'std::array' cannot hold C arrays of an unknown size");
+
   using __trivially_relocatable = __conditional_t<__libcpp_is_trivially_relocatable<_Tp>::value, array, void>;
 
   // types:

libcxx/include/deque

@@ -220,6 +220,7 @@ template <class T, class Allocator, class Predicate>
 #include <__ranges/size.h>
 #include <__split_buffer>
 #include <__type_traits/conditional.h>
+#include <__type_traits/diagnostic_utilities.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_allocator.h>
 #include <__type_traits/is_convertible.h>
@@ -481,6 +482,8 @@ const _DiffType __deque_iterator<_ValueType, _Pointer, _Reference, _MapPointer,
 
 template <class _Tp, class _Allocator /*= allocator<_Tp>*/>
 class _LIBCPP_TEMPLATE_VIS deque {
+  static_assert(__container_requirements<deque, _Tp>::value);
+
 public:
   // types:
 
@@ -2609,6 +2612,8 @@ inline constexpr bool __format::__enable_insertable<std::deque<wchar_t>> = true;
 
 #endif // _LIBCPP_STD_VER >= 20
 
+_LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS(deque);
+
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 17

libcxx/include/forward_list

@@ -218,6 +218,7 @@ template <class T, class Allocator, class Predicate>
 #include <__ranges/container_compatible_range.h>
 #include <__ranges/from_range.h>
 #include <__type_traits/conditional.h>
+#include <__type_traits/diagnostic_utilities.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_allocator.h>
 #include <__type_traits/is_const.h>
@@ -478,6 +479,8 @@ public:
 
 template <class _Tp, class _Alloc>
 class __forward_list_base {
+  static_assert(__container_requirements<forward_list, _Tp>::value);
+
 protected:
   typedef _Tp value_type;
   typedef _Alloc allocator_type;
@@ -589,6 +592,8 @@ private:
   }
 };
 
+_LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS(forward_list);
+
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _Tp, class _Alloc>

libcxx/include/list

@@ -225,8 +225,10 @@ template <class T, class Allocator, class Predicate>
 #include <__ranges/container_compatible_range.h>
 #include <__ranges/from_range.h>
 #include <__type_traits/conditional.h>
+#include <__type_traits/diagnostic_utilities.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_allocator.h>
+#include <__type_traits/is_const.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/is_pointer.h>
@@ -465,6 +467,8 @@ public:
 
 template <class _Tp, class _Alloc>
 class __list_imp {
+  static_assert(__container_requirements<list, _Tp>::value);
+
 public:
   __list_imp(const __list_imp&)            = delete;
   __list_imp& operator=(const __list_imp&) = delete;
@@ -591,6 +595,8 @@ private:
   _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(__list_imp&, false_type) _NOEXCEPT {}
 };
 
+_LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS(list);
+
 // Unlink nodes [__f, __l]
 template <class _Tp, class _Alloc>
 inline void __list_imp<_Tp, _Alloc>::__unlink_nodes(__base_pointer __f, __base_pointer __l) _NOEXCEPT {

libcxx/include/map

@@ -595,6 +595,11 @@ erase_if(multimap<Key, T, Compare, Allocator>& c, Predicate pred);  // C++20
 #include <__ranges/from_range.h>
 #include <__tree>
 #include <__type_traits/is_allocator.h>
+#include <__type_traits/is_array.h>
+#include <__type_traits/is_function.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_unbounded_array.h>
+#include <__type_traits/is_void.h>
 #include <__type_traits/remove_const.h>
 #include <__type_traits/type_identity.h>
 #include <__utility/forward.h>
@@ -969,6 +974,13 @@ public:
 
 template <class _Key, class _Tp, class _Compare = less<_Key>, class _Allocator = allocator<pair<const _Key, _Tp> > >
 class _LIBCPP_TEMPLATE_VIS map {
+  // TODO(#106635): replace with _LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS
+  // Remember to remove relevant headers when this is completed.
+  static_assert(!is_lvalue_reference<_Key>::value, "'std::map' cannot hold references");
+  static_assert(!is_function<_Key>::value && !is_function<_Tp>::value, "'std::map' cannot hold functions");
+  static_assert(!is_void<_Key>::value && !is_void<_Tp>::value, "'std::map' cannot hold 'void'");
+  static_assert(!__libcpp_is_unbounded_array<_Key>::value, "'std::map' cannot hold C arrays of an unknown size");
+
 public:
   // types:
   typedef _Key key_type;
@@ -1646,6 +1658,13 @@ erase_if(map<_Key, _Tp, _Compare, _Allocator>& __c, _Predicate __pred) {
 
 template <class _Key, class _Tp, class _Compare = less<_Key>, class _Allocator = allocator<pair<const _Key, _Tp> > >
 class _LIBCPP_TEMPLATE_VIS multimap {
+  // TODO(#106635): replace with _LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS
+  // Remember to remove relevant headers when this is completed.
+  static_assert(!is_lvalue_reference<_Key>::value, "'std::multimap' cannot hold references");
+  static_assert(!is_function<_Key>::value && !is_function<_Tp>::value, "'std::multimap' cannot hold functions");
+  static_assert(!is_void<_Key>::value && !is_void<_Tp>::value, "'std::multimap' cannot hold 'void'");
+  static_assert(!__libcpp_is_unbounded_array<_Key>::value, "'std::multimap' cannot hold C arrays of an unknown size");
+
 public:
   // types:
   typedef _Key key_type;

libcxx/include/module.modulemap

@@ -1494,7 +1494,17 @@ module std_private_memory_align                           [system] { header "__m
 module std_private_memory_aligned_alloc                   [system] { header "__memory/aligned_alloc.h" }
 module std_private_memory_allocate_at_least               [system] { header "__memory/allocate_at_least.h" }
 module std_private_memory_allocation_guard                [system] { header "__memory/allocation_guard.h" }
-module std_private_memory_allocator                       [system] { header "__memory/allocator.h" }
+module std_private_memory_allocator                       [system] {
+  header "__memory/allocator.h"
+  export std_private_type_traits_diagnostic_utilities
+  export std_private_type_traits_is_bounded_array
+  export std_private_type_traits_is_const
+  export std_private_type_traits_is_function
+  export std_private_type_traits_is_reference
+  export std_private_type_traits_is_unbounded_array
+  export std_private_type_traits_is_void
+  export std_private_type_traits_is_volatile
+}
 module std_private_memory_allocator_arg_t                 [system] { header "__memory/allocator_arg_t.h" }
 module std_private_memory_allocator_destructor            [system] { header "__memory/allocator_destructor.h" }
 module std_private_memory_allocator_traits                [system] { header "__memory/allocator_traits.h" }
@@ -1871,6 +1881,10 @@ module std_private_type_traits_decay                                     [system
 }
 module std_private_type_traits_dependent_type                            [system] { header "__type_traits/dependent_type.h" }
 module std_private_type_traits_desugars_to                               [system] { header "__type_traits/desugars_to.h" }
+module std_private_type_traits_diagnostic_utilities [system] {
+  textual header "__type_traits/diagnostic_utilities.h"
+  export *
+}
 module std_private_type_traits_disjunction                               [system] { header "__type_traits/disjunction.h" }
 module std_private_type_traits_enable_if                                 [system] { header "__type_traits/enable_if.h" }
 module std_private_type_traits_extent                                    [system] { header "__type_traits/extent.h" }

libcxx/include/set

@@ -531,6 +531,7 @@ erase_if(multiset<Key, Compare, Allocator>& c, Predicate pred);  // C++20
 #include <__ranges/container_compatible_range.h>
 #include <__ranges/from_range.h>
 #include <__tree>
+#include <__type_traits/diagnostic_utilities.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_allocator.h>
 #include <__type_traits/is_nothrow_assignable.h>
@@ -570,6 +571,8 @@ class multiset;
 
 template <class _Key, class _Compare = less<_Key>, class _Allocator = allocator<_Key> >
 class _LIBCPP_TEMPLATE_VIS set {
+  static_assert(__container_requirements<set, _Key>::value);
+
 public:
   // types:
   typedef _Key key_type;
@@ -900,6 +903,8 @@ public:
 #endif
 };
 
+_LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS(set);
+
 #if _LIBCPP_STD_VER >= 17
 template <class _InputIterator,
           class _Compare   = less<__iter_value_type<_InputIterator>>,
@@ -1024,6 +1029,8 @@ erase_if(set<_Key, _Compare, _Allocator>& __c, _Predicate __pred) {
 
 template <class _Key, class _Compare = less<_Key>, class _Allocator = allocator<_Key> >
 class _LIBCPP_TEMPLATE_VIS multiset {
+  static_assert(__container_requirements<multiset, _Key>::value);
+
 public:
   // types:
   typedef _Key key_type;
@@ -1357,6 +1364,8 @@ public:
 #endif
 };
 
+_LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS(multiset);
+
 #if _LIBCPP_STD_VER >= 17
 template <class _InputIterator,
           class _Compare   = less<__iter_value_type<_InputIterator>>,

libcxx/include/string

@@ -621,6 +621,7 @@ basic_string<char32_t> operator""s( const char32_t *str, size_t len );
 #include <__string/char_traits.h>
 #include <__string/extern_template_lists.h>
 #include <__type_traits/conditional.h>
+#include <__type_traits/diagnostic_utilities.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_allocator.h>
 #include <__type_traits/is_array.h>
@@ -760,6 +761,13 @@ struct __padding<0> {};
 template <class _CharT, class _Traits, class _Allocator>
 class basic_string {
 private:
+  // This is_array check precedes _LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS since basic_string
+  // never permits arrays, and earlier static_asserts suppress later ones (meaning that this one is
+  // always emitted for both 'basic_string<char[]>' and 'basic_string<char[10]>', and doesn't say
+  // "before C++20").
+  static_assert(!is_array<_CharT>::value, "'std::basic_string' cannot hold C arrays");
+  static_assert(__container_requirements<basic_string, _CharT>::value);
+
   using __default_allocator_type = allocator<_CharT>;
 
 public:
@@ -823,7 +831,6 @@ public:
 #  define _LIBCPP_ASAN_VOLATILE_WRAPPER(PTR) PTR
 #endif
 
-  static_assert(!is_array<value_type>::value, "Character type of basic_string must not be an array");
   static_assert(is_standard_layout<value_type>::value, "Character type of basic_string must be standard-layout");
   static_assert(is_trivial<value_type>::value, "Character type of basic_string must be trivial");
   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
@@ -2260,6 +2267,8 @@ private:
   operator==(const basic_string<_CharT2, _Traits2, _Allocator2>&, const _CharT2*) _NOEXCEPT;
 };
 
+_LIBCPP_DEFINE__CONTAINER_VALUE_TYPE_REQUIREMENTS(basic_string);
+
 // These declarations must appear before any functions are implicitly used
 // so that they have the correct visibility specifier.
 #define _LIBCPP_DECLARE(...) extern template __VA_ARGS__;