Re: [PATCH] c: Implement C23 nullptr (N3042)

[Jason Merrill <jason@redhat.com>]

On 8/13/22 14:35, Marek Polacek wrote:
> This patch implements the C23 nullptr literal:
> <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3042.htm>, which is
> intended to replace the problematic definition of NULL which might be
> either of integer type or void*.
> 
> Since C++ has had nullptr for over a decade now, it was relatively easy
> to just move the built-in node definitions from the C++ FE to the C/C++
> common code.  Also, our DWARF emitter already handles NULLPTR_TYPE by
> emitting DW_TAG_unspecified_type.  However, I had to handle a lot of
> contexts such as ?:, comparison, conversion, etc.
> 
> There are some minor differences, e.g. in C you can do
> 
>    bool b = nullptr;
> 
> but in C++ you have to use direct-initialization:
> 
>    bool b{nullptr};
> 
> And I think that
> 
>    nullptr_t n = 0;
> 
> is only valid in C++.
> 
> Of course, C doesn't have to handle mangling, RTTI, substitution,
> overloading, ...
> 
> This patch also defines nullptr_t in <stddef.h>.  I'm uncertain about
> the __STDC_VERSION__ version I should be checking.  Also, I'm not
> defining __STDC_VERSION_STDDEF_H__ yet, because I don't know what value
> it should be defined to.  Do we know yet?
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

The C++ changes are OK, but you probably want a comment in 
c_common_nodes_and_builtins that we aren't setting the alignment there 
for C++ backward ABI bug compatibility.  Or perhaps set it there and 
then break it in the C++ front end when abi < 9.

> gcc/c-family/ChangeLog:
> 
> 	* c-common.cc (c_common_reswords): Enable nullptr in C.
> 	(c_common_nodes_and_builtins): Create the built-in node for nullptr.
> 	* c-common.h (enum c_tree_index): Add CTI_NULLPTR, CTI_NULLPTR_TYPE.
> 	(nullptr_node): Define.
> 	(nullptr_type_node): Define.
> 	(NULLPTR_TYPE_P): Define.
> 	* c-pretty-print.cc (c_pretty_printer::simple_type_specifier): Handle
> 	NULLPTR_TYPE.
> 	(c_pretty_printer::direct_abstract_declarator): Likewise.
> 	(c_pretty_printer::constant): Likewise.
> 
> gcc/c/ChangeLog:
> 
> 	* c-convert.cc (c_convert) <case POINTER_TYPE>: Handle NULLPTR_TYPE.
> 	Give a better diagnostic when converting to nullptr_t.
> 	* c-decl.cc (c_init_decl_processing): Perform C-specific nullptr
> 	initialization.
> 	* c-parser.cc (c_parser_postfix_expression): Handle RID_NULLPTR.
> 	* c-typeck.cc (null_pointer_constant_p): Return true for NULLPTR_TYPE_P.
> 	(build_unary_op) <case TRUTH_NOT_EXPR>: Handle NULLPTR_TYPE.
> 	(build_conditional_expr): Handle the case when the second/third operand
> 	is NULLPTR_TYPE and third/second operand is POINTER_TYPE.
> 	(convert_for_assignment): Handle converting an expression of type
> 	nullptr_t to pointer/bool.
> 	(build_binary_op) <case TRUTH_XOR_EXPR>: Handle NULLPTR_TYPE.
> 	<case EQ_EXPR>: Likewise.
> 
> gcc/cp/ChangeLog:
> 
> 	* cp-tree.h (enum cp_tree_index): Remove CTI_NULLPTR, CTI_NULLPTR_TYPE.
> 	Move it to c_tree_index.
> 	(nullptr_node): No longer define here.
> 	(nullptr_type_node): Likewise.
> 	(NULLPTR_TYPE_P): Likewise.
> 	* decl.cc (cxx_init_decl_processing): Only keep C++-specific nullptr
> 	initialization; move the shared code to c_common_nodes_and_builtins.
> 
> gcc/ChangeLog:
> 
> 	* ginclude/stddef.h: Define nullptr_t.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.dg/Wcxx-compat-2.c: Remove nullptr test.
> 	* gcc.dg/c2x-nullptr-1.c: New test.
> 	* gcc.dg/c2x-nullptr-2.c: New test.
> 	* gcc.dg/c2x-nullptr-3.c: New test.
> 	* gcc.dg/c2x-nullptr-4.c: New test.
> 	* gcc.dg/c2x-nullptr-5.c: New test.
> ---
>   gcc/c-family/c-common.cc             |  13 +-
>   gcc/c-family/c-common.h              |   8 +
>   gcc/c-family/c-pretty-print.cc       |   7 +
>   gcc/c/c-convert.cc                   |  19 ++-
>   gcc/c/c-decl.cc                      |   6 +
>   gcc/c/c-parser.cc                    |   8 +
>   gcc/c/c-typeck.cc                    |  55 +++++-
>   gcc/cp/cp-tree.h                     |   8 -
>   gcc/cp/decl.cc                       |   8 +-
>   gcc/ginclude/stddef.h                |   8 +
>   gcc/testsuite/gcc.dg/Wcxx-compat-2.c |   1 -
>   gcc/testsuite/gcc.dg/c2x-nullptr-1.c | 239 +++++++++++++++++++++++++++
>   gcc/testsuite/gcc.dg/c2x-nullptr-2.c |   9 +
>   gcc/testsuite/gcc.dg/c2x-nullptr-3.c |  62 +++++++
>   gcc/testsuite/gcc.dg/c2x-nullptr-4.c |  10 ++
>   gcc/testsuite/gcc.dg/c2x-nullptr-5.c |  11 ++
>   16 files changed, 448 insertions(+), 24 deletions(-)
>   create mode 100644 gcc/testsuite/gcc.dg/c2x-nullptr-1.c
>   create mode 100644 gcc/testsuite/gcc.dg/c2x-nullptr-2.c
>   create mode 100644 gcc/testsuite/gcc.dg/c2x-nullptr-3.c
>   create mode 100644 gcc/testsuite/gcc.dg/c2x-nullptr-4.c
>   create mode 100644 gcc/testsuite/gcc.dg/c2x-nullptr-5.c
> 
> diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
> index 6e41ceb38e9..809e7ff5804 100644
> --- a/gcc/c-family/c-common.cc
> +++ b/gcc/c-family/c-common.cc
> @@ -500,7 +500,7 @@ const struct c_common_resword c_common_reswords[] =
>     { "namespace",	RID_NAMESPACE,	D_CXXONLY | D_CXXWARN },
>     { "new",		RID_NEW,	D_CXXONLY | D_CXXWARN },
>     { "noexcept",		RID_NOEXCEPT,	D_CXXONLY | D_CXX11 | D_CXXWARN },
> -  { "nullptr",		RID_NULLPTR,	D_CXXONLY | D_CXX11 | D_CXXWARN },
> +  { "nullptr",		RID_NULLPTR,	D_CXX11 | D_CXXWARN },
>     { "operator",		RID_OPERATOR,	D_CXXONLY | D_CXXWARN },
>     { "private",		RID_PRIVATE,	D_CXX_OBJC | D_CXXWARN },
>     { "protected",	RID_PROTECTED,	D_CXX_OBJC | D_CXXWARN },
> @@ -4723,6 +4723,17 @@ c_common_nodes_and_builtins (void)
>     null_node = make_int_cst (1, 1);
>     TREE_TYPE (null_node) = c_common_type_for_size (POINTER_SIZE, 0);
>   
> +  /* Create the built-in nullptr node.  This part of its initialization is
> +     common to C and C++.  The front ends can further adjust its definition
> +     in {c,cxx}_init_decl_processing.  */
> +  nullptr_type_node = make_node (NULLPTR_TYPE);
> +  TYPE_SIZE (nullptr_type_node) = bitsize_int (GET_MODE_BITSIZE (ptr_mode));
> +  TYPE_SIZE_UNIT (nullptr_type_node) = size_int (GET_MODE_SIZE (ptr_mode));
> +  TYPE_UNSIGNED (nullptr_type_node) = 1;
> +  TYPE_PRECISION (nullptr_type_node) = GET_MODE_BITSIZE (ptr_mode);
> +  SET_TYPE_MODE (nullptr_type_node, ptr_mode);
> +  nullptr_node = build_int_cst (nullptr_type_node, 0);
> +
>     /* Since builtin_types isn't gc'ed, don't export these nodes.  */
>     memset (builtin_types, 0, sizeof (builtin_types));
>   }
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index c06769b6f0b..d30174334c2 100644
> --- a/gcc/c-family/c-common.h
> +++ b/gcc/c-family/c-common.h
> @@ -375,6 +375,8 @@ enum c_tree_index
>       CTI_DEFAULT_FUNCTION_TYPE,
>   
>       CTI_NULL,
> +    CTI_NULLPTR,
> +    CTI_NULLPTR_TYPE,
>   
>       /* These are not types, but we have to look them up all the time.  */
>       CTI_FUNCTION_NAME_DECL,
> @@ -534,6 +536,9 @@ extern const unsigned int num_c_common_reswords;
>   
>   /* The node for C++ `__null'.  */
>   #define null_node                       c_global_trees[CTI_NULL]
> +/* The nodes for `nullptr'.  */
> +#define nullptr_node                    c_global_trees[CTI_NULLPTR]
> +#define nullptr_type_node               c_global_trees[CTI_NULLPTR_TYPE]
>   
>   extern GTY(()) tree c_global_trees[CTI_MAX];
>   
> @@ -1009,6 +1014,9 @@ extern void c_parse_final_cleanups (void);
>   #define DECL_UNNAMED_BIT_FIELD(NODE) \
>     (DECL_C_BIT_FIELD (NODE) && !DECL_NAME (NODE))
>   
> +/* True iff TYPE is cv decltype(nullptr).  */
> +#define NULLPTR_TYPE_P(TYPE) (TREE_CODE (TYPE) == NULLPTR_TYPE)
> +
>   extern tree do_case (location_t, tree, tree);
>   extern tree build_stmt (location_t, enum tree_code, ...);
>   extern tree build_real_imag_expr (location_t, enum tree_code, tree);
> diff --git a/gcc/c-family/c-pretty-print.cc b/gcc/c-family/c-pretty-print.cc
> index 71a0cb51093..efa1768f4d6 100644
> --- a/gcc/c-family/c-pretty-print.cc
> +++ b/gcc/c-family/c-pretty-print.cc
> @@ -321,6 +321,7 @@ pp_c_pointer (c_pretty_printer *pp, tree t)
>         _Bool                          -- C99
>         _Complex                       -- C99
>         _Imaginary                     -- C99
> +      nullptr_t                      -- C23
>         struct-or-union-specifier
>         enum-specifier
>         typedef-name.
> @@ -424,6 +425,9 @@ c_pretty_printer::simple_type_specifier (tree t)
>         else
>   	translate_string ("<anonymous>");
>         break;
> +    case NULLPTR_TYPE:
> +      pp_c_ws_string (this, "nullptr_t");
> +      break;
>   
>       default:
>         pp_unsupported_tree (this, t);
> @@ -678,6 +682,7 @@ c_pretty_printer::direct_abstract_declarator (tree t)
>       case COMPLEX_TYPE:
>       case TYPE_DECL:
>       case ERROR_MARK:
> +    case NULLPTR_TYPE:
>         break;
>   
>       default:
> @@ -1219,6 +1224,8 @@ c_pretty_printer::constant (tree e)
>   	  pp_c_character_constant (this, e);
>   	else if (TREE_CODE (type) == ENUMERAL_TYPE)
>   	  pp_c_enumeration_constant (this, e);
> +	else if (NULLPTR_TYPE_P (type))
> +	  pp_string (this, "nullptr");
>   	else
>   	  pp_c_integer_constant (this, e);
>         }
> diff --git a/gcc/c/c-convert.cc b/gcc/c/c-convert.cc
> index 18083d59618..013fe6b2a53 100644
> --- a/gcc/c/c-convert.cc
> +++ b/gcc/c/c-convert.cc
> @@ -133,6 +133,14 @@ c_convert (tree type, tree expr, bool init_const)
>   	(loc, type, c_objc_common_truthvalue_conversion (input_location, expr));
>   
>       case POINTER_TYPE:
> +      /* The type nullptr_t may be converted to a pointer type.  The result is
> +	 a null pointer value.  */
> +      if (NULLPTR_TYPE_P (TREE_TYPE (e)))
> +	{
> +	  ret = build_int_cst (type, 0);
> +	  goto maybe_fold;
> +	}
> +      gcc_fallthrough ();
>       case REFERENCE_TYPE:
>         ret = convert_to_pointer (type, e);
>         goto maybe_fold;
> @@ -180,7 +188,16 @@ c_convert (tree type, tree expr, bool init_const)
>         return ret;
>       }
>   
> -  error ("conversion to non-scalar type requested");
> +  /* If we are converting to nullptr_t, don't say "non-scalar type" because
> +     the nullptr_t type is a scalar type.  Only nullptr_t shall be converted
> +     to nullptr_t.  */
> +  if (code == NULLPTR_TYPE)
> +    {
> +      error ("conversion from %qT to %qT", TREE_TYPE (e), type);
> +      inform (input_location, "only %qT can be converted to %qT", type, type);
> +    }
> +  else
> +    error ("conversion to non-scalar type requested");
>     return error_mark_node;
>   }
>   
> diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
> index ae8990c138f..ac4394c9dc5 100644
> --- a/gcc/c/c-decl.cc
> +++ b/gcc/c/c-decl.cc
> @@ -4531,6 +4531,12 @@ c_init_decl_processing (void)
>     pushdecl (build_decl (UNKNOWN_LOCATION, TYPE_DECL, get_identifier ("_Bool"),
>   			boolean_type_node));
>   
> +  /* C-specific nullptr initialization.  */
> +  record_builtin_type (RID_MAX, "nullptr_t", nullptr_type_node);
> +  /* The size and alignment of nullptr_t is the same as for a pointer to
> +     character type.  */
> +  SET_TYPE_ALIGN (nullptr_type_node, GET_MODE_ALIGNMENT (ptr_mode));
> +
>     input_location = save_loc;
>   
>     make_fname_decl = c_make_fname_decl;
> diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
> index 759f200a7eb..2af3a614fb9 100644
> --- a/gcc/c/c-parser.cc
> +++ b/gcc/c/c-parser.cc
> @@ -10243,6 +10243,14 @@ c_parser_postfix_expression (c_parser *parser)
>   			 "%<depend%> clause");
>   	  expr.set_error ();
>   	  break;
> +	/* C23 'nullptr' literal.  */
> +	case RID_NULLPTR:
> +	  c_parser_consume_token (parser);
> +	  expr.value = nullptr_node;
> +	  set_c_expr_source_range (&expr, tok_range);
> +	  pedwarn_c11 (loc, OPT_Wpedantic,
> +		       "ISO C does not support %qs before C2X", "nullptr");
> +	  break;
>   	default:
>   	  c_parser_error (parser, "expected expression");
>   	  expr.set_error ();
> diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
> index d37de2a313b..2ba48345a1b 100644
> --- a/gcc/c/c-typeck.cc
> +++ b/gcc/c/c-typeck.cc
> @@ -133,6 +133,13 @@ null_pointer_constant_p (const_tree expr)
>     /* This should really operate on c_expr structures, but they aren't
>        yet available everywhere required.  */
>     tree type = TREE_TYPE (expr);
> +
> +  /* An integer constant expression with the value 0, such an expression
> +     cast to type void*, or the predefined constant nullptr, are a null
> +     pointer constant.  */
> +  if (NULLPTR_TYPE_P (type))
> +    return true;
> +
>     return (TREE_CODE (expr) == INTEGER_CST
>   	  && !TREE_OVERFLOW (expr)
>   	  && integer_zerop (expr)
> @@ -4575,7 +4582,7 @@ build_unary_op (location_t location, enum tree_code code, tree xarg,
>       case TRUTH_NOT_EXPR:
>         if (typecode != INTEGER_TYPE && typecode != FIXED_POINT_TYPE
>   	  && typecode != REAL_TYPE && typecode != POINTER_TYPE
> -	  && typecode != COMPLEX_TYPE)
> +	  && typecode != COMPLEX_TYPE && typecode != NULLPTR_TYPE)
>   	{
>   	  error_at (location,
>   		    "wrong type argument to unary exclamation mark");
> @@ -5515,6 +5522,13 @@ build_conditional_expr (location_t colon_loc, tree ifexp, bool ifexp_bcp,
>   	}
>         result_type = type2;
>       }
> +  /* 6.5.15: "if one is a null pointer constant (other than a pointer) or has
> +     type nullptr_t and the other is a pointer, the result type is the pointer
> +     type."  */
> +  else if (code1 == NULLPTR_TYPE && code2 == POINTER_TYPE)
> +    result_type = type2;
> +  else if (code1 == POINTER_TYPE && code2 == NULLPTR_TYPE)
> +    result_type = type1;
>   
>     if (!result_type)
>       {
> @@ -7613,9 +7627,10 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
>   	error_at (location, msg);
>         return error_mark_node;
>       }
> -  else if (codel == POINTER_TYPE && coder == INTEGER_TYPE)
> +  else if (codel == POINTER_TYPE
> +	   && (coder == INTEGER_TYPE || coder == NULLPTR_TYPE))
>       {
> -      /* An explicit constant 0 can convert to a pointer,
> +      /* An explicit constant 0 or nullptr can convert to a pointer,
>   	 or one that results from arithmetic, even including
>   	 a cast to integer type.  */
>         if (!null_pointer_constant)
> @@ -7691,7 +7706,10 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
>   
>         return convert (type, rhs);
>       }
> -  else if (codel == BOOLEAN_TYPE && coder == POINTER_TYPE)
> +  else if (codel == BOOLEAN_TYPE
> +	   /* The type nullptr_t may be converted to bool.  The
> +	      result is false.  */
> +	   && (coder == POINTER_TYPE || coder == NULLPTR_TYPE))
>       {
>         tree ret;
>         bool save = in_late_binary_op;
> @@ -12107,10 +12125,10 @@ build_binary_op (location_t location, enum tree_code code,
>       case TRUTH_XOR_EXPR:
>         if ((code0 == INTEGER_TYPE || code0 == POINTER_TYPE
>   	   || code0 == REAL_TYPE || code0 == COMPLEX_TYPE
> -	   || code0 == FIXED_POINT_TYPE)
> +	   || code0 == FIXED_POINT_TYPE || code0 == NULLPTR_TYPE)
>   	  && (code1 == INTEGER_TYPE || code1 == POINTER_TYPE
>   	      || code1 == REAL_TYPE || code1 == COMPLEX_TYPE
> -	      || code1 == FIXED_POINT_TYPE))
> +	      || code1 == FIXED_POINT_TYPE || code1 ==  NULLPTR_TYPE))
>   	{
>   	  /* Result of these operations is always an int,
>   	     but that does not mean the operands should be
> @@ -12418,6 +12436,31 @@ build_binary_op (location_t location, enum tree_code code,
>   	  result_type = type1;
>   	  pedwarn (location, 0, "comparison between pointer and integer");
>   	}
> +      else if (null_pointer_constant_p (orig_op0)
> +	       && null_pointer_constant_p (orig_op1))
> +	{
> +	  /* 6.5.9: One of the following shall hold:
> +	      -- both operands have type nullptr_t;  */
> +	  if (code0 == NULLPTR_TYPE && code1 == NULLPTR_TYPE)
> +	    {
> +	      result_type = nullptr_type_node;
> +	      /* No need to convert the operands to result_type later.  */
> +	      converted = 1;
> +	    }
> +	  /* -- one operand has type nullptr_t and the other is a null pointer
> +	     constant.  We will have to convert the former to the type of the
> +	     latter, because during gimplification we can't have mismatching
> +	     comparison operand type.  We convert from nullptr_t to the other
> +	     type, since only nullptr_t can be converted to nullptr_t.  Also,
> +	     even a constant 0 is a null pointer constant, so we may have to
> +	     create a pointer type from its type.  */
> +	  else if (code0 == NULLPTR_TYPE)
> +	    result_type = (INTEGRAL_TYPE_P (type1)
> +			   ? build_pointer_type (type1) : type1);
> +	  else
> +	    result_type = (INTEGRAL_TYPE_P (type0)
> +			   ? build_pointer_type (type0) : type0);
> +	}
>         if ((TREE_CODE (TREE_TYPE (orig_op0)) == BOOLEAN_TYPE
>   	   || truth_value_p (TREE_CODE (orig_op0)))
>   	  ^ (TREE_CODE (TREE_TYPE (orig_op1)) == BOOLEAN_TYPE
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 3278b4114bd..eb461bf8374 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -187,9 +187,6 @@ enum cp_tree_index
>       CPTI_NOEXCEPT_FALSE_SPEC,
>       CPTI_NOEXCEPT_DEFERRED_SPEC,
>   
> -    CPTI_NULLPTR,
> -    CPTI_NULLPTR_TYPE,
> -
>       CPTI_ANY_TARG,
>   
>       CPTI_MODULE_HWM,
> @@ -254,8 +251,6 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
>   #define conv_op_marker			cp_global_trees[CPTI_CONV_OP_MARKER]
>   #define abort_fndecl			cp_global_trees[CPTI_ABORT_FNDECL]
>   #define current_aggr			cp_global_trees[CPTI_AGGR_TAG]
> -#define nullptr_node			cp_global_trees[CPTI_NULLPTR]
> -#define nullptr_type_node		cp_global_trees[CPTI_NULLPTR_TYPE]
>   /* std::align_val_t */
>   #define align_type_node			cp_global_trees[CPTI_ALIGN_TYPE]
>   
> @@ -4405,9 +4400,6 @@ get_vec_init_expr (tree t)
>      || TREE_CODE (TYPE) == REAL_TYPE \
>      || TREE_CODE (TYPE) == COMPLEX_TYPE)
>   
> -/* True iff TYPE is cv decltype(nullptr).  */
> -#define NULLPTR_TYPE_P(TYPE) (TREE_CODE (TYPE) == NULLPTR_TYPE)
> -
>   /* [basic.types]
>   
>      Arithmetic types, enumeration types, pointer types,
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index ff56fddba54..ba4fb21d36a 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -4793,16 +4793,10 @@ cxx_init_decl_processing (void)
>   	  }
>         }
>   
> -    nullptr_type_node = make_node (NULLPTR_TYPE);
> -    TYPE_SIZE (nullptr_type_node) = bitsize_int (GET_MODE_BITSIZE (ptr_mode));
> -    TYPE_SIZE_UNIT (nullptr_type_node) = size_int (GET_MODE_SIZE (ptr_mode));
> -    TYPE_UNSIGNED (nullptr_type_node) = 1;
> -    TYPE_PRECISION (nullptr_type_node) = GET_MODE_BITSIZE (ptr_mode);
> +    /* C++-specific nullptr initialization.  */
>       if (abi_version_at_least (9))
>         SET_TYPE_ALIGN (nullptr_type_node, GET_MODE_ALIGNMENT (ptr_mode));
> -    SET_TYPE_MODE (nullptr_type_node, ptr_mode);
>       record_builtin_type (RID_MAX, "decltype(nullptr)", nullptr_type_node);
> -    nullptr_node = build_int_cst (nullptr_type_node, 0);
>     }
>   
>     if (! supports_one_only ())
> diff --git a/gcc/ginclude/stddef.h b/gcc/ginclude/stddef.h
> index 79e296d4a66..2ccf411ed88 100644
> --- a/gcc/ginclude/stddef.h
> +++ b/gcc/ginclude/stddef.h
> @@ -443,6 +443,14 @@ typedef struct {
>   #endif
>   #endif /* C++11.  */
>   
> +#if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 202000L)
> +#ifndef _GCC_NULLPTR_T
> +#define _GCC_NULLPTR_T
> +  typedef __typeof__(nullptr) nullptr_t;
> +/* ??? This doesn't define __STDC_VERSION_STDDEF_H__ yet.  */
> +#endif
> +#endif /* C23.  */
> +
>   #endif /* _STDDEF_H was defined this time */
>   
>   #endif /* !_STDDEF_H && !_STDDEF_H_ && !_ANSI_STDDEF_H && !__STDDEF_H__
> diff --git a/gcc/testsuite/gcc.dg/Wcxx-compat-2.c b/gcc/testsuite/gcc.dg/Wcxx-compat-2.c
> index 4578bece109..61d33a0b028 100644
> --- a/gcc/testsuite/gcc.dg/Wcxx-compat-2.c
> +++ b/gcc/testsuite/gcc.dg/Wcxx-compat-2.c
> @@ -18,7 +18,6 @@ int friend;			/* { dg-warning "5:keyword" } */
>   int mutable;			/* { dg-warning "5:keyword" } */
>   int namespace;			/* { dg-warning "5:keyword" } */
>   int new;			/* { dg-warning "5:keyword" } */
> -int nullptr;			/* { dg-warning "5:keyword" } */
>   int operator;			/* { dg-warning "5:keyword" } */
>   int private;			/* { dg-warning "5:keyword" } */
>   int protected;			/* { dg-warning "5:keyword" } */
> diff --git a/gcc/testsuite/gcc.dg/c2x-nullptr-1.c b/gcc/testsuite/gcc.dg/c2x-nullptr-1.c
> new file mode 100644
> index 00000000000..49e7031c4d9
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/c2x-nullptr-1.c
> @@ -0,0 +1,239 @@
> +/* Test basic usage of C23 nullptr.  */
> +/* { dg-do run } */
> +/* { dg-options "-std=c2x -pedantic-errors -Wall -Wextra -Wno-unused-variable" } */
> +
> +#include <stdarg.h>
> +
> +typedef __typeof__(nullptr) nullptr_t;
> +
> +void f1 (nullptr_t) { }
> +void f2 (int *) { }
> +void f3 (_Bool) { }
> +nullptr_t cmp (void) { return nullptr; }
> +
> +/* The type nullptr_t shall not be converted to any type other than bool or
> +   a pointer type.  No type other than nullptr_t shall be converted to nullptr_t.  */
> +void
> +test1 (void)
> +{
> +  const nullptr_t nptr = nullptr;
> +  static nullptr_t static_nptr;
> +  int *p1 = nullptr;
> +  void *p2 = nullptr;
> +  float *p3 = nullptr;
> +  void (*p4)(int) = nullptr;
> +  int (*p5)[10] = nullptr;
> +  int *p6 = nptr;
> +  void *p7 = nptr;
> +  float *p8 = nptr;
> +  void (*p9)(int) = nptr;
> +  int (*p10)[10] = nptr;
> +  int *p11 = (int *) nullptr;
> +  int *p12 = (int *) nptr;
> +  if (nullptr || p1 || p2 || p3 || p4 || p5 || p6 || p7 || p8 || p9 || p10
> +      || p11 || p12)
> +    __builtin_abort ();
> +
> +  _Bool b1 = nullptr;
> +  _Bool b2 = (_Bool) nullptr;
> +  _Bool b3 = nptr;
> +  _Bool b4 = (_Bool) nptr;
> +  if (b1 || b2 || b3 || b4 || (_Bool) nullptr || (_Bool) nptr)
> +   __builtin_abort ();
> +
> +  __auto_type a1 = nullptr;
> +  __auto_type a2 = nptr;
> +
> +  /* We can convert nullptr_t to nullptr_t.  */
> +  __typeof__(nullptr) x = nullptr;
> +  f1 (x);
> +  f1 (nullptr);
> +  f2 (x);
> +  f2 (nullptr);
> +  f3 (nullptr);
> +
> +  const nullptr_t np1 = nullptr;
> +  const nullptr_t np2 = np1;
> +}
> +
> +/* Test valid comparison.  */
> +void
> +test2 (int *p)
> +{
> +  /* If both operands have type nullptr_t or one operand has type nullptr_t
> +     and the other is a null pointer constant, they compare equal.  */
> +  const nullptr_t nptr = nullptr;
> +  int r = 0;
> +
> +  r |= nullptr != nullptr;
> +  r |= cmp () != nullptr;
> +  r |= nullptr != cmp ();
> +  r |= !(nullptr == nullptr);
> +  r |= !(cmp () == nullptr);
> +  r |= !(nullptr == cmp ());
> +  r |= nullptr != (void *) 0;
> +  r |= !(nullptr == (void *) 0);
> +  r |= (void *) 0 != nullptr;
> +  r |= !((void *) 0 == nullptr);
> +  r |= nullptr != 0;
> +  r |= 0 != nullptr;
> +  r |= !(nullptr == 0);
> +  r |= !(0 == nullptr);
> +  r |= nullptr != 0u;
> +  r |= 0u != nullptr;
> +  r |= !(nullptr == 0u);
> +  r |= !(0u == nullptr);
> +
> +  r |= nptr != nptr;
> +  r |= cmp () != nptr;
> +  r |= nptr != cmp ();
> +  r |= !(nptr == nptr);
> +  r |= !(cmp () == nptr);
> +  r |= !(nptr == cmp ());
> +  r |= nptr != (void *) 0;
> +  r |= !(nptr == (void *) 0);
> +  r |= (void *) 0 != nptr;
> +  r |= !((void *) 0 == nptr);
> +  r |= nptr != 0;
> +  r |= 0 != nptr;
> +  r |= !(nptr == 0);
> +  r |= !(0 == nptr);
> +  r |= nptr != 0u;
> +  r |= 0u != nptr;
> +  r |= !(nptr == 0u);
> +  r |= !(0u == nptr);
> +  if (r)
> +    __builtin_abort ();
> +
> +  (void) (p == nullptr);
> +  (void) (p != nullptr);
> +  (void) (nullptr == p);
> +  (void) (nullptr != p);
> +}
> +
> +/* Test ?:.  */
> +void
> +test3 (int *p, _Bool b)
> +{
> +  int x = nullptr ? 1 : 2;
> +  (void) x;
> +  const nullptr_t nptr = nullptr;
> +  /* One of the following shall hold for the second and third operands:
> +     -- both operands have nullptr_t type.  */
> +  __auto_type r1 = b ? nullptr : nullptr;
> +  __auto_type r2 = b ? nptr : nptr;
> +  /* -- one operand is a pointer and the other is a null pointer constant
> +     or has type nullptr_t;  */
> +  __auto_type r3 = b ? p : nullptr;
> +  __auto_type r4 = b ? nullptr : p;
> +  __auto_type r5 = b ? nptr : p;
> +  __auto_type r6 = b ? p : nptr;
> +  __auto_type r7 = b ? 0 : p;
> +  __auto_type r8 = b ? p : 0;
> +  __auto_type r9 = b ? p : cmp ();
> +  __auto_type r10 = b ?  cmp () : p;
> +}
> +
> +/* Simple assignment.  */
> +void
> +test4 (void)
> +{
> +  /* -- the left operand has an atomic, qualified, or unqualified version of
> +     the nullptr_t type and the type of the right is nullptr_t;  */
> +  nullptr_t n1;
> +  n1 = nullptr;
> +  const nullptr_t n2 = nullptr;
> +  _Atomic nullptr_t n3 = nullptr;
> +  volatile nullptr_t n4 = nullptr;
> +  /* -- the left operand is an atomic, qualified, or unqualified pointer,
> +     and the type of the right is nullptr_t;  */
> +  int *p1 = cmp ();
> +  _Atomic int *p2 = cmp ();
> +  const int *volatile p3 = cmp ();
> +  const int *const *const p4 = cmp ();
> +  double (*const p5)(void) = n1;
> +  /* -- the left operand is an atomic, qualified, or unqualified bool, and
> +     the type of the right is nullptr_t;  */
> +  _Bool b1;
> +  b1 = cmp ();
> +  const _Bool b2 = nullptr;
> +  _Atomic _Bool b3;
> +  b3 = n1;
> +  (void) b1;
> +  (void) b3;
> +}
> +
> +/* var_arg etc.  */
> +static void
> +test5 (int i, ...)
> +{
> +  va_list ap;
> +  va_start (ap, i);
> +  if (va_arg (ap, void *))
> +    __builtin_abort ();
> +}
> +
> +/* Operand of alignas, sizeof or typeof operators.  */
> +void
> +test6 (void)
> +{
> +  _Static_assert (sizeof (nullptr) == sizeof (void *), "sizeof (nullptr)");
> +  _Static_assert (sizeof (nullptr_t) == sizeof (void *), "sizeof (nullptr_t)");
> +  _Static_assert (sizeof (nullptr) == sizeof (char *), "sizeof (nullptr)");
> +  _Static_assert (sizeof (nullptr_t) == sizeof (char *), "sizeof (nullptr_t)");
> +  _Static_assert (_Alignof (nullptr_t) == _Alignof (char *), "_Alignof (nullptr_t)");
> +  __typeof__(nullptr) t = nullptr;
> +  f1 (t);
> +  _Alignas (nullptr_t) char i1 = 'q';
> +
> +  _Static_assert (_Generic (nullptr, nullptr_t: 1, default: 0) == 1, "_Generic");
> +  _Static_assert (_Generic (t, nullptr_t: 1, default: 0) == 1, "_Generic");
> +  _Static_assert (_Generic (cmp (), nullptr_t: 1, default: 0) == 1, "_Generic");
> +  _Static_assert (_Generic (0, nullptr_t: 1, int: 2, default: 0) == 2, "_Generic");
> +  _Static_assert (_Generic ((void *)0, nullptr_t: 1, void *: 2, default: 0) == 2, "_Generic");
> +  _Static_assert (_Generic (nullptr, nullptr_t: 1, void *: 2, default: 0) == 1, "_Generic");
> +}
> +
> +/* Play with !, ||, &&. */
> +void
> +test7 (void)
> +{
> +  if (nullptr)
> +    __builtin_abort ();
> +  if (1 && nullptr)
> +    __builtin_abort ();
> +  if (0 || nullptr)
> +    __builtin_abort ();
> +  if (nullptr && 1)
> +    __builtin_abort ();
> +  if (nullptr || 0)
> +    __builtin_abort ();
> +  if (!nullptr)
> +    {
> +    }
> +  else
> +    __builtin_abort ();
> +  while (nullptr)
> +    __builtin_abort ();
> +  int i = 0;
> +  do
> +    ++i;
> +  while (nullptr);
> +  if (i != 1)
> +    __builtin_abort ();
> +  for (;nullptr;)
> +    __builtin_abort ();
> +}
> +
> +int
> +main (void)
> +{
> +  int i = 42;
> +  test1 ();
> +  test2 (&i);
> +  test3 (&i, 0);
> +  test4 ();
> +  test5 (42, nullptr);
> +  test6 ();
> +  test7 ();
> +}
> diff --git a/gcc/testsuite/gcc.dg/c2x-nullptr-2.c b/gcc/testsuite/gcc.dg/c2x-nullptr-2.c
> new file mode 100644
> index 00000000000..2cc88353581
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/c2x-nullptr-2.c
> @@ -0,0 +1,9 @@
> +/* Test nullptr_t from <stddef.h..  */
> +/* { dg-do compile } */
> +/* { dg-options "-std=c2x -pedantic-errors" } */
> +
> +#include <stddef.h>
> +
> +void f(nullptr_t);
> +_Static_assert (sizeof (nullptr_t) == sizeof (char *), "sizeof (nullptr_t)");
> +_Static_assert (_Alignof (nullptr_t) == _Alignof (char *), "_Alignof (nullptr_t)");
> diff --git a/gcc/testsuite/gcc.dg/c2x-nullptr-3.c b/gcc/testsuite/gcc.dg/c2x-nullptr-3.c
> new file mode 100644
> index 00000000000..3f37c9b17ed
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/c2x-nullptr-3.c
> @@ -0,0 +1,62 @@
> +/* Test wrong usage of C23 nullptr.  */
> +/* { dg-do compile } */
> +/* { dg-options "-std=c2x -pedantic-errors -Wall -Wextra -Wno-unused-variable" } */
> +
> +typedef __typeof__(nullptr) nullptr_t;
> +
> +void g (nullptr_t); /* { dg-message "expected .nullptr_t. but argument is of type .int." } */
> +
> +void
> +test1 (int *p)
> +{
> +  (void) (p > nullptr); /* { dg-error "ordered comparison" } */
> +  (void) (p >= nullptr); /* { dg-error "ordered comparison" } */
> +  (void) (p < nullptr); /* { dg-error "ordered comparison" } */
> +  (void) (p <= nullptr); /* { dg-error "ordered comparison" } */
> +  (void) (nullptr == 1); /* { dg-error "invalid operands" } */
> +  (void) (1 == nullptr); /* { dg-error "invalid operands" } */
> +  (void) (nullptr != 1); /* { dg-error "invalid operands" } */
> +  (void) (1 != nullptr); /* { dg-error "invalid operands" } */
> +  (void) (1 > nullptr); /* { dg-error "invalid operands" } */
> +}
> +
> +void
> +test2 (void)
> +{
> +  const nullptr_t nptr = nullptr;
> +  int p = nullptr; /* { dg-error "incompatible types" } */
> +  float d = nullptr; /* { dg-error "incompatible types" } */
> +  char arr[10] = { nullptr }; /* { dg-error "incompatible types" } */
> +
> +  /* No type other than nullptr_t shall be converted to nullptr_t.  */
> +  const nullptr_t n = 0; /* { dg-error "invalid initializer" } */
> +  +(nullptr_t) 0; /* { dg-error "conversion from .int. to .nullptr_t." } */
> +
> +  g (0); /* { dg-error "incompatible type" } */
> +
> +  int i = 42 + nullptr; /* { dg-error "invalid operands" } */
> +
> +  /* The assignment of an object of type nullptr_t with a value of another
> +     type, even if the value is a null pointer constant, is a constraint
> +     violation.  */
> +  nullptr_t m;
> +  m = 0; /* { dg-error "incompatible types" } */
> +  (void) m;
> +  nullptr_t o = 0; /* { dg-error "invalid initializer" } */
> +
> +  switch (nullptr); /* { dg-error "switch quantity not an integer" } */
> +}
> +
> +/* If a second or third operand of type nullptr_t is used that is not a null
> +   pointer constant and the other operand is not a pointer or does not have
> +   itself nullptr_t, a constraint is violated even if that other operand is
> +   a null pointer constant such as 0.  */
> +void
> +test3 (_Bool b, int i)
> +{
> +  const nullptr_t nptr = nullptr;
> +  __auto_type a1 = b ? nptr : i; /* { dg-error "type mismatch" } */
> +  __auto_type a2 = b ? i : nptr; /* { dg-error "type mismatch" } */
> +  __auto_type a3 = b ? nptr : 0; /* { dg-error "type mismatch" } */
> +  __auto_type a4 = b ? 0 : nptr; /* { dg-error "type mismatch" } */
> +}
> diff --git a/gcc/testsuite/gcc.dg/c2x-nullptr-4.c b/gcc/testsuite/gcc.dg/c2x-nullptr-4.c
> new file mode 100644
> index 00000000000..5b15e75d159
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/c2x-nullptr-4.c
> @@ -0,0 +1,10 @@
> +/* Test that we warn about `nullptr' pre-C2X.  */
> +/* { dg-do compile } */
> +/* { dg-options "-std=c17 -pedantic-errors" } */
> +
> +int *
> +fn (int *p)
> +{
> +  p = nullptr; /* { dg-error "ISO C does not support .nullptr. before C2X" } */
> +  return p;
> +}
> diff --git a/gcc/testsuite/gcc.dg/c2x-nullptr-5.c b/gcc/testsuite/gcc.dg/c2x-nullptr-5.c
> new file mode 100644
> index 00000000000..7479ab4ea1d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/c2x-nullptr-5.c
> @@ -0,0 +1,11 @@
> +/* Test that -Wc11-c2x-compat issues a warning (not a pedwarn) about
> +   `nullptr' in C2X.  */
> +/* { dg-do compile } */
> +/* { dg-options "-std=c2x -pedantic-errors -Wc11-c2x-compat" } */
> +
> +int *
> +fn (int *p)
> +{
> +  p = nullptr; /* { dg-warning "ISO C does not support .nullptr. before C2X" } */
> +  return p;
> +}
> 
> base-commit: 4991e20923b658ce9fbdf5621cab39f71b98fbc2