[gcc r14-2146] c++: C++26 constexpr cast from void* [PR110344]

[gcc r14-2146] c++: C++26 constexpr cast from void* [PR110344]

[Jason Merrill]

https://gcc.gnu.org/g:a1c6e9631ca33990a2b7411060ca4d18db081a7d

commit r14-2146-ga1c6e9631ca33990a2b7411060ca4d18db081a7d
Author: Jason Merrill <jason@redhat.com>
Date:   Sat Jun 24 05:15:02 2023 -0400

    c++: C++26 constexpr cast from void* [PR110344]
    
    P2768 allows static_cast from void* to ob* in constant evaluation if the
    pointer does in fact point to an object of the appropriate type.
    cxx_fold_indirect_ref already does the work of finding such an object if it
    happens to be a subobject rather than the outermost object at that address,
    as in constexpr-voidptr2.C.
    
            P2768
            PR c++/110344
    
    gcc/c-family/ChangeLog:
    
            * c-cppbuiltin.cc (c_cpp_builtins): Update __cpp_constexpr.
    
    gcc/cp/ChangeLog:
    
            * constexpr.cc (cxx_eval_constant_expression): In C++26, allow cast
            from void* to the type of a pointed-to object.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp26/constexpr-voidptr1.C: New test.
            * g++.dg/cpp26/constexpr-voidptr2.C: New test.
            * g++.dg/cpp26/feat-cxx26.C: New test.

Diff:
---
 gcc/c-family/c-cppbuiltin.cc                    |   8 +-
 gcc/cp/constexpr.cc                             |  11 +
 gcc/testsuite/g++.dg/cpp26/constexpr-voidptr1.C |  35 ++
 gcc/testsuite/g++.dg/cpp26/constexpr-voidptr2.C |  15 +
 gcc/testsuite/g++.dg/cpp26/feat-cxx26.C         | 597 ++++++++++++++++++++++++
 5 files changed, 665 insertions(+), 1 deletion(-)

diff --git a/gcc/c-family/c-cppbuiltin.cc b/gcc/c-family/c-cppbuiltin.cc
index 5d64625fcd7..6bd4c1261a7 100644
--- a/gcc/c-family/c-cppbuiltin.cc
+++ b/gcc/c-family/c-cppbuiltin.cc
@@ -1075,12 +1075,18 @@ c_cpp_builtins (cpp_reader *pfile)
 	  cpp_define (pfile, "__cpp_size_t_suffix=202011L");
 	  cpp_define (pfile, "__cpp_if_consteval=202106L");
 	  cpp_define (pfile, "__cpp_auto_cast=202110L");
-	  cpp_define (pfile, "__cpp_constexpr=202211L");
+	  if (cxx_dialect <= cxx23)
+	    cpp_define (pfile, "__cpp_constexpr=202211L");
 	  cpp_define (pfile, "__cpp_multidimensional_subscript=202211L");
 	  cpp_define (pfile, "__cpp_named_character_escapes=202207L");
 	  cpp_define (pfile, "__cpp_static_call_operator=202207L");
 	  cpp_define (pfile, "__cpp_implicit_move=202207L");
 	}
+      if (cxx_dialect > cxx23)
+	{
+	  /* Set feature test macros for C++26.  */
+	  cpp_define (pfile, "__cpp_constexpr=202306L");
+	}
       if (flag_concepts)
         {
 	  if (cxx_dialect >= cxx20)
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 432b3a275e8..cca0435bafc 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -7681,6 +7681,17 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    && !is_std_construct_at (ctx->call)
 	    && !is_std_allocator_allocate (ctx->call))
 	  {
+	    /* P2738 (C++26): a conversion from a prvalue P of type "pointer to
+	       cv void" to a pointer-to-object type T unless P points to an
+	       object whose type is similar to T.  */
+	    if (cxx_dialect > cxx23)
+	      if (tree ob
+		  = cxx_fold_indirect_ref (ctx, loc, TREE_TYPE (type), op))
+		{
+		  r = build1 (ADDR_EXPR, type, ob);
+		  break;
+		}
+
 	    /* Likewise, don't error when casting from void* when OP is
 	       &heap uninit and similar.  */
 	    tree sop = tree_strip_nop_conversions (op);
diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr1.C b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr1.C
new file mode 100644
index 00000000000..ce0ccbef5f9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr1.C
@@ -0,0 +1,35 @@
+// PR c++/110344
+// { dg-do compile { target c++26 } }
+
+#include <string_view>
+struct Sheep {
+  constexpr std::string_view speak() const noexcept { return "Baaaaaa"; }
+};
+struct Cow {
+  constexpr std::string_view speak() const noexcept { return "Mooo"; }
+};
+class Animal_View {
+private:
+  const void *animal;
+  std::string_view (*speak_function)(const void *);
+public:
+  template <typename Animal>
+  constexpr Animal_View(const Animal &a)
+    : animal{&a}, speak_function{[](const void *object) {
+				   return static_cast<const Animal *>(object)->speak();
+				 }} {}
+  constexpr std::string_view speak() const noexcept {
+    return speak_function(animal);
+  }
+};
+// This is the key bit here. This is a single concrete function
+// that can take anything that happens to have the "Animal_View"
+// interface
+constexpr std::string_view do_speak(Animal_View av) { return av.speak(); }
+int main() {
+  // A Cow is a cow. The only think that makes it special
+  // is that it has a "std::string_view speak() const" member
+  constexpr Cow cow;
+  constexpr auto result = do_speak(cow);
+  return static_cast<int>(result.size());
+}
diff --git a/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr2.C b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr2.C
new file mode 100644
index 00000000000..e746301e9f9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/constexpr-voidptr2.C
@@ -0,0 +1,15 @@
+// PR c++/110344
+// { dg-do compile { target c++26 } }
+
+struct A { int i; };
+struct B { A a; };
+
+constexpr int f()
+{
+  B b { 42 };
+  void *p = &b;
+  A* ap = static_cast<A*>(p);
+  return ap->i;
+}
+
+static_assert (f() == 42);
diff --git a/gcc/testsuite/g++.dg/cpp26/feat-cxx26.C b/gcc/testsuite/g++.dg/cpp26/feat-cxx26.C
new file mode 100644
index 00000000000..0977d964fe0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/feat-cxx26.C
@@ -0,0 +1,597 @@
+// { dg-options "-std=c++26 -I${srcdir}/g++.dg/cpp1y -I${srcdir}/g++.dg/cpp1y/testinc" }
+
+//  C++98 features:
+
+#ifndef __cpp_rtti
+#  error "__cpp_rtti"
+#elif  __cpp_rtti != 199711
+#  error "__cpp_rtti != 199711"
+#endif
+
+#ifndef __cpp_exceptions
+#  error "__cpp_exceptions"
+#elif  __cpp_exceptions != 199711
+#  error "__cpp_exceptions != 199711"
+#endif
+
+//  C++11 features:
+
+#ifndef __cpp_raw_strings
+#  error "__cpp_raw_strings"
+#elif __cpp_raw_strings != 200710
+#  error "__cpp_raw_strings != 200710"
+#endif
+
+#ifndef __cpp_unicode_literals
+#  error "__cpp_unicode_literals"
+#elif __cpp_unicode_literals != 200710
+#  error "__cpp_unicode_literals != 200710"
+#endif
+
+#ifndef __cpp_user_defined_literals
+#  error "__cpp_user_defined_literals"
+#elif __cpp_user_defined_literals != 200809
+#  error "__cpp_user_defined_literals != 200809"
+#endif
+
+#ifndef __cpp_lambdas
+#  error "__cpp_lambdas"
+#elif __cpp_lambdas != 200907
+#  error "__cpp_lambdas != 200907"
+#endif
+
+#ifndef __cpp_range_based_for
+#  error "__cpp_range_based_for"
+#elif __cpp_range_based_for != 201603
+#  error "__cpp_range_based_for != 201603"
+#endif
+
+#ifndef __cpp_decltype
+#  error "__cpp_decltype"
+#elif __cpp_decltype != 200707
+#  error "__cpp_decltype != 200707"
+#endif
+
+#ifndef __cpp_attributes
+#  error "__cpp_attributes"
+#elif __cpp_attributes != 200809
+#  error "__cpp_attributes != 200809"
+#endif
+
+#ifndef __cpp_rvalue_references
+#  error "__cpp_rvalue_references"
+#elif __cpp_rvalue_references != 200610
+#  error "__cpp_rvalue_references != 200610"
+#endif
+
+#ifndef __cpp_variadic_templates
+#  error "__cpp_variadic_templates"
+#elif __cpp_variadic_templates != 200704
+#  error "__cpp_variadic_templates != 200704"
+#endif
+
+#ifndef __cpp_initializer_lists
+#  error "__cpp_initializer_lists"
+#elif __cpp_initializer_lists != 200806
+#  error "__cpp_initializer_lists != 200806"
+#endif
+
+#ifndef __cpp_delegating_constructors
+#  error "__cpp_delegating_constructors"
+#elif __cpp_delegating_constructors != 200604
+#  error "__cpp_delegating_constructors != 200604"
+#endif
+
+#ifndef __cpp_nsdmi
+#  error "__cpp_nsdmi"
+#elif __cpp_nsdmi != 200809
+#  error "__cpp_nsdmi != 200809"
+#endif
+
+#ifndef __cpp_inheriting_constructors
+#  error "__cpp_inheriting_constructors"
+#elif  __cpp_inheriting_constructors!= 201511
+#  error "__cpp_inheriting_constructors != 201511"
+#endif
+
+#ifndef __cpp_ref_qualifiers
+#  error "__cpp_ref_qualifiers"
+#elif __cpp_ref_qualifiers != 200710
+#  error "__cpp_ref_qualifiers != 200710"
+#endif
+
+#ifndef __cpp_alias_templates
+#  error "__cpp_alias_templates"
+#elif __cpp_alias_templates != 200704
+#  error "__cpp_alias_templates != 200704"
+#endif
+
+#ifndef __cpp_threadsafe_static_init
+#  error "__cpp_threadsafe_static_init"
+#elif __cpp_threadsafe_static_init != 200806
+#  error "__cpp_threadsafe_static_init != 200806"
+#endif
+
+//  C++14 features:
+
+#ifndef __cpp_binary_literals
+#  error "__cpp_binary_literals"
+#elif __cpp_binary_literals != 201304
+#  error "__cpp_binary_literals != 201304"
+#endif
+
+#ifndef __cpp_init_captures
+#  error "__cpp_init_captures"
+#elif __cpp_init_captures != 201803
+#  error "__cpp_init_captures != 201803"
+#endif
+
+#ifndef __cpp_generic_lambdas
+#  error "__cpp_generic_lambdas"
+#elif __cpp_generic_lambdas != 201707
+#  error "__cpp_generic_lambdas != 201707"
+#endif
+
+#ifndef __cpp_constexpr
+#  error "__cpp_constexpr"
+#elif __cpp_constexpr != 202306L
+#  error "__cpp_constexpr != 202306L"
+#endif
+
+#ifndef __cpp_decltype_auto
+#  error "__cpp_decltype_auto"
+#elif __cpp_decltype_auto != 201304
+#  error "__cpp_decltype_auto != 201304"
+#endif
+
+#ifndef __cpp_return_type_deduction
+#  error "__cpp_return_type_deduction"
+#elif __cpp_return_type_deduction != 201304
+#  error "__cpp_return_type_deduction != 201304"
+#endif
+
+#ifndef __cpp_aggregate_nsdmi
+#  error "__cpp_aggregate_nsdmi"
+#elif __cpp_aggregate_nsdmi != 201304
+#  error "__cpp_aggregate_nsdmi != 201304"
+#endif
+
+#ifndef __cpp_variable_templates
+#  error "__cpp_variable_templates"
+#elif __cpp_variable_templates != 201304
+#  error "__cpp_variable_templates != 201304"
+#endif
+
+#ifndef __cpp_digit_separators
+#  error "__cpp_digit_separators"
+#elif __cpp_digit_separators != 201309
+#  error "__cpp_digit_separators != 201309"
+#endif
+
+#ifndef __cpp_sized_deallocation
+#  error "__cpp_sized_deallocation"
+#elif __cpp_sized_deallocation != 201309
+#  error "__cpp_sized_deallocation != 201309"
+#endif
+
+//  GNU VLA support:
+
+#ifndef __cpp_runtime_arrays
+#  error "__cpp_runtime_arrays"
+#elif __cpp_runtime_arrays != 198712
+#  error "__cpp_runtime_arrays != 198712"
+#endif
+
+//  C++11 attributes:
+
+#ifdef __has_cpp_attribute
+#  if ! __has_cpp_attribute(noreturn)
+#    error "__has_cpp_attribute(noreturn)"
+#  elif __has_cpp_attribute(noreturn) != 200809
+#    error "__has_cpp_attribute(noreturn) != 200809"
+#  endif
+#else
+#  error "__has_cpp_attribute"
+#endif
+
+//  Attribute carries_dependency not in yet.
+//#ifdef __has_cpp_attribute
+//#  if ! __has_cpp_attribute(carries_dependency)
+//#    error "__has_cpp_attribute(carries_dependency)"
+//#  elif __has_cpp_attribute(carries_dependency) != 200809
+//#    error "__has_cpp_attribute(carries_dependency) != 200809"
+//#  endif
+//#else
+//#  error "__has_cpp_attribute"
+//#endif
+
+//  C++14 attributes:
+
+#ifdef __has_cpp_attribute
+#  if ! __has_cpp_attribute(deprecated)
+#    error "__has_cpp_attribute(deprecated)"
+#  elif __has_cpp_attribute(deprecated) != 201309
+#    error "__has_cpp_attribute(deprecated) != 201309"
+#  endif
+#else
+#  error "__has_cpp_attribute"
+#endif
+
+//  Include checks:
+
+//  Check for __has_include macro.
+#ifndef __has_include
+#  error "__has_include"
+#endif
+
+//  Try known bracket header (use operator).
+#if __has_include (<complex>)
+#else
+#  error "<complex>"
+#endif
+
+//  Define and use a macro to invoke the operator.
+#define sluggo(TXT) __has_include(TXT)
+
+#if sluggo(<complex>)
+#else
+#  error "<complex>"
+#endif
+
+#if ! sluggo(<complex>)
+#  error "<complex>"
+#else
+#endif
+
+//  Quoted complex.h should find at least the bracket version.
+#if __has_include("complex.h")
+#else
+#  error "complex.h"
+#endif
+
+//  Try known local quote header.
+#if __has_include("complex_literals.h")
+#else
+#  error "\"complex_literals.h\""
+#endif
+
+//  Try nonexistent bracket header.
+#if __has_include(<stuff>)
+#  error "<stuff>"
+#else
+#endif
+
+//  Try nonexistent quote header.
+#if __has_include("phlegm")
+#  error "\"phlegm\""
+#else
+#endif
+
+//  Test __has_include_next.
+#if __has_include("phoobhar.h")
+#  include "phoobhar.h"
+#else
+#  error "__has_include(\"phoobhar.h\")"
+#endif
+
+//  Try a macro.
+#define COMPLEX_INC "complex.h"
+#if __has_include(COMPLEX_INC)
+#else
+#  error COMPLEX_INC
+#endif
+
+//  Realistic use of __has_include.
+#if __has_include(<array>)
+#  define STD_ARRAY 1
+#  include <array>
+  template<typename _Tp, std::size_t _Num>
+    using array = std::array<_Tp, _Num>;
+#elif __has_include(<tr1/array>)
+#  define TR1_ARRAY 1
+#  include <tr1/array>
+  template<typename _Tp, std::size_t _Num>
+    typedef std::tr1::array<_Tp, _Num> array;
+#endif
+
+// C++17 features:
+
+#ifndef __cpp_unicode_characters
+#  error "__cpp_unicode_characters"
+#elif __cpp_unicode_characters != 201411
+#  error "__cpp_unicode_characters != 201411"
+#endif
+
+#ifndef __cpp_static_assert
+#  error "__cpp_static_assert"
+#elif __cpp_static_assert != 201411
+#  error "__cpp_static_assert != 201411"
+#endif
+
+#ifndef __cpp_namespace_attributes
+#  error "__cpp_namespace_attributes"
+#elif __cpp_namespace_attributes != 201411
+#  error "__cpp_namespace_attributes != 201411"
+#endif
+
+#ifndef __cpp_enumerator_attributes
+#  error "__cpp_enumerator_attributes"
+#elif __cpp_enumerator_attributes != 201411
+#  error "__cpp_enumerator_attributes != 201411"
+#endif
+
+#ifndef __cpp_nested_namespace_definitions
+#  error "__cpp_nested_namespace_definitions"
+#elif __cpp_nested_namespace_definitions != 201411
+#  error "__cpp_nested_namespace_definitions != 201411"
+#endif
+
+#ifndef __cpp_fold_expressions
+#  error "__cpp_fold_expressions"
+#elif __cpp_fold_expressions != 201603
+#  error "__cpp_fold_expressions != 201603"
+#endif
+
+#ifndef __cpp_nontype_template_args
+#  error "__cpp_nontype_template_args"
+#elif __cpp_nontype_template_args != 201911
+#  error "__cpp_nontype_template_args != 201911"
+#endif
+
+#ifndef __cpp_hex_float
+#  error "__cpp_hex_float"
+#elif __cpp_hex_float != 201603
+#  error "__cpp_hex_float != 201603"
+#endif
+
+#ifndef __cpp_aggregate_bases
+#  error "__cpp_aggregate_bases"
+#elif __cpp_aggregate_bases != 201603
+#  error "__cpp_aggregate_bases != 201603"
+#endif
+
+#ifndef __cpp_deduction_guides
+#  error "__cpp_deduction_guides"
+#elif __cpp_deduction_guides != 201907
+#  error "__cpp_deduction_guides != 201907"
+#endif
+
+#ifndef __cpp_if_constexpr
+#  error "__cpp_if_constexpr"
+#elif __cpp_if_constexpr != 201606
+#  error "__cpp_if_constexpr != 201606"
+#endif
+
+#ifndef __cpp_aligned_new
+#  error "__cpp_aligned_new"
+#elif __cpp_aligned_new != 201606
+#  error "__cpp_aligned_new != 201606"
+#endif
+
+#ifndef __cpp_template_auto
+#  error "__cpp_template_auto"
+#elif __cpp_template_auto != 201606
+#  error "__cpp_template_auto != 201606"
+#endif
+
+#ifndef __cpp_inline_variables
+#  error "__cpp_inline_variables"
+#elif __cpp_inline_variables != 201606
+#  error "__cpp_inline_variables != 201606"
+#endif
+
+#ifndef __cpp_capture_star_this
+#  error "__cpp_capture_star_this"
+#elif __cpp_capture_star_this != 201603
+#  error "__cpp_capture_star_this != 201603"
+#endif
+
+#ifndef __cpp_noexcept_function_type
+#  error "__cpp_noexcept_function_type"
+#elif __cpp_noexcept_function_type != 201510
+#  error "__cpp_noexcept_function_type != 201510"
+#endif
+
+#ifndef __cpp_structured_bindings
+#  error "__cpp_structured_bindings"
+#elif __cpp_structured_bindings != 201606
+#  error "__cpp_structured_bindings != 201606"
+#endif
+
+#ifndef __cpp_template_template_args
+#  error "__cpp_template_template_args"
+#elif __cpp_template_template_args != 201611
+#  error "__cpp_template_template_args != 201611"
+#endif
+
+#ifndef __cpp_variadic_using
+#  error "__cpp_variadic_using"
+#elif __cpp_variadic_using != 201611
+#  error "__cpp_variadic_using != 201611"
+#endif
+
+#ifndef __cpp_guaranteed_copy_elision
+#  error "__cpp_guaranteed_copy_elision"
+#elif __cpp_guaranteed_copy_elision != 201606
+#  error "__cpp_guaranteed_copy_elision != 201606"
+#endif
+
+#ifndef __cpp_nontype_template_parameter_auto
+#  error "__cpp_nontype_template_parameter_auto"
+#elif __cpp_nontype_template_parameter_auto != 201606
+#  error "__cpp_nontype_template_parameter_auto != 201606"
+#endif
+
+// C++20 features:
+
+#ifndef __cpp_conditional_explicit
+#  error "__cpp_conditional_explicit"
+#elif __cpp_conditional_explicit != 201806
+#  error "__cpp_conditional_explicit != 201806"
+#endif
+
+#ifndef __cpp_nontype_template_parameter_class
+#  error "__cpp_nontype_template_parameter_class"
+#elif __cpp_nontype_template_parameter_class != 201806
+#  error "__cpp_nontype_template_parameter_class != 201806"
+#endif
+
+#ifndef __cpp_impl_destroying_delete
+#  error "__cpp_impl_destroying_delete"
+#elif __cpp_impl_destroying_delete != 201806
+#  error "__cpp_impl_destroying_delete != 201806"
+#endif
+
+#ifndef __cpp_constinit
+#  error "__cpp_constinit"
+#elif __cpp_constinit != 201907
+#  error "__cpp_constinit != 201907"
+#endif
+
+#ifndef __cpp_constexpr_dynamic_alloc
+#  error "__cpp_constexpr_dynamic_alloc"
+#elif __cpp_constexpr_dynamic_alloc != 201907
+#  error "__cpp_constexpr_dynamic_alloc != 201907"
+#endif
+
+#ifndef __cpp_aggregate_paren_init
+#  error "__cpp_aggregate_paren_init"
+#elif __cpp_aggregate_paren_init != 201902
+#  error "__cpp_aggregate_paren_init != 201902"
+#endif
+
+#ifndef __cpp_char8_t
+#  error "__cpp_char8_t"
+#elif __cpp_char8_t != 202207
+#  error "__cpp_char8_t != 202207"
+#endif
+
+#ifndef __cpp_designated_initializers
+#  error "__cpp_designated_initializers"
+#elif __cpp_designated_initializers != 201707
+#  error "__cpp_designated_initializers != 201707"
+#endif
+
+#ifndef __cpp_constexpr_in_decltype
+#  error "__cpp_constexpr_in_decltype"
+#elif __cpp_constexpr_in_decltype != 201711
+#  error "__cpp_constexpr_in_decltype != 201711"
+#endif
+
+#ifndef __cpp_consteval
+#  error "__cpp_consteval"
+#elif __cpp_consteval != 201811
+#  error "__cpp_consteval != 201811"
+#endif
+
+#ifndef __cpp_concepts
+#  error "__cpp_concepts"
+#elif __cpp_concepts != 202002
+#  error "__cpp_concepts != 202002"
+#endif
+
+#ifndef __cpp_using_enum
+#  error "__cpp_using_enum"
+#elif __cpp_using_enum != 201907
+#  error "__cpp_using_enum != 201907"
+#endif
+
+// C++20 attributes:
+
+#ifdef __has_cpp_attribute
+
+#  if ! __has_cpp_attribute(maybe_unused)
+#    error "__has_cpp_attribute(maybe_unused)"
+#  elif __has_cpp_attribute(maybe_unused) != 201603
+#    error "__has_cpp_attribute(maybe_unused) != 201603"
+#  endif
+
+#  if ! __has_cpp_attribute(nodiscard)
+#    error "__has_cpp_attribute(nodiscard)"
+#  elif __has_cpp_attribute(nodiscard) != 201907
+#    error "__has_cpp_attribute(nodiscard) != 201907"
+#  endif
+
+#  if ! __has_cpp_attribute(fallthrough)
+#    error "__has_cpp_attribute(fallthrough)"
+#  elif __has_cpp_attribute(fallthrough) != 201603
+#    error "__has_cpp_attribute(fallthrough) != 201603"
+#  endif
+
+#  if ! __has_cpp_attribute(no_unique_address)
+#    error "__has_cpp_attribute(no_unique_address)"
+#  elif __has_cpp_attribute(no_unique_address) != 201803
+#    error "__has_cpp_attribute(no_unique_address) != 201803"
+#  endif
+
+#  if ! __has_cpp_attribute(likely)
+#    error "__has_cpp_attribute(likely)"
+#  elif __has_cpp_attribute(likely) != 201803
+#    error "__has_cpp_attribute(likely) != 201803"
+#  endif
+
+#  if ! __has_cpp_attribute(unlikely)
+#    error "__has_cpp_attribute(unlikely)"
+#  elif __has_cpp_attribute(unlikely) != 201803
+#    error "__has_cpp_attribute(unlikely) != 201803"
+#  endif
+
+#else
+#  error "__has_cpp_attribute"
+#endif
+
+// C++23 features:
+
+#ifndef __cpp_size_t_suffix
+#  error "__cpp_size_t_suffix"
+#elif __cpp_size_t_suffix != 202011
+#  error "__cpp_size_t_suffix != 202011"
+#endif
+
+#ifndef __cpp_if_consteval
+#  error "__cpp_if_consteval"
+#elif __cpp_if_consteval != 202106
+#  error "__cpp_if_consteval != 202106"
+#endif
+
+#ifndef __cpp_multidimensional_subscript
+#  error "__cpp_multidimensional_subscript"
+#elif __cpp_multidimensional_subscript != 202211
+#  error "__cpp_multidimensional_subscript != 202211"
+#endif
+
+#ifndef __cpp_named_character_escapes
+#  error "__cpp_named_character_escapes"
+#elif __cpp_named_character_escapes != 202207
+#  error "__cpp_named_character_escapes != 202207"
+#endif
+
+#ifndef __cpp_static_call_operator
+#  error "__cpp_static_call_operator"
+#elif __cpp_static_call_operator != 202207
+#  error "__cpp_static_call_operator != 202207"
+#endif
+
+#ifndef __cpp_implicit_move
+#  error "__cpp_implicit_move"
+#elif __cpp_implicit_move != 202207
+#  error "__cpp_implicit_move != 202207"
+#endif
+
+#ifndef __cpp_auto_cast
+#  error "__cpp_auto_cast"
+#elif __cpp_auto_cast != 202110
+#  error "__cpp_auto_cast != 202110"
+#endif
+
+//  C++23 attributes:
+
+#ifdef __has_cpp_attribute
+#  if ! __has_cpp_attribute(assume)
+#    error "__has_cpp_attribute(assume)"
+#  elif __has_cpp_attribute(assume) != 202207
+#    error "__has_cpp_attribute(assume) != 202207"
+#  endif
+#else
+#  error "__has_cpp_attribute"
+#endif