c: special-case some "bool" errors with C23 (v2) [PR117629]

Changed in v2:
- distinguish between "bool" and "_Bool" when determining
  standard version

This patch attempts to provide better error messages for
code compiled with C23 that hasn't been updated for
"bool", "true", and "false" becoming keywords.

Specifically:

(1) with "typedef int bool;" previously we emitted:

t1.c:7:13: error: two or more data types in declaration specifiers
    7 | typedef int bool;
      |             ^~~~
t1.c:7:1: warning: useless type name in empty declaration
    7 | typedef int bool;
      | ^~~~~~~

whereas with this patch we emit:

t1.c:7:13: error: 'bool' cannot be defined via 'typedef'
    7 | typedef int bool;
      |             ^~~~
t1.c:7:13: note: 'bool' is a keyword with '-std=c23' onwards
t1.c:7:1: warning: useless type name in empty declaration
    7 | typedef int bool;
      | ^~~~~~~

(2) with "int bool;" previously we emitted:

t2.c:7:5: error: two or more data types in declaration specifiers
    7 | int bool;
      |     ^~~~
t2.c:7:1: warning: useless type name in empty declaration
    7 | int bool;
      | ^~~

whereas with this patch we emit:

t2.c:7:5: error: 'bool' cannot be used here
    7 | int bool;
      |     ^~~~
t2.c:7:5: note: 'bool' is a keyword with '-std=c23' onwards
t2.c:7:1: warning: useless type name in empty declaration
    7 | int bool;
      | ^~~

(3) with "typedef enum { false = 0, true = 1 } _Bool;" previously we
emitted:

t3.c:7:16: error: expected identifier before 'false'
    7 | typedef enum { false = 0, true = 1 } _Bool;
      |                ^~~~~
t3.c:7:38: error: expected ';', identifier or '(' before '_Bool'
    7 | typedef enum { false = 0, true = 1 } _Bool;
      |                                      ^~~~~
t3.c:7:38: warning: useless type name in empty declaration

whereas with this patch we emit:

t3.c:7:16: error: cannot use keyword 'false' as enumeration constant
    7 | typedef enum { false = 0, true = 1 } _Bool;
      |                ^~~~~
t3.c:7:16: note: 'false' is a keyword with '-std=c23' onwards
t3.c:7:38: error: expected ';', identifier or '(' before '_Bool'
    7 | typedef enum { false = 0, true = 1 } _Bool;
      |                                      ^~~~~
t3.c:7:38: warning: useless type name in empty declaration

gcc/c/ChangeLog:
	PR c/117629
	* c-decl.cc (declspecs_add_type): Special-case attempts to use
	bool as a typedef name or declaration name.
	* c-errors.cc (get_std_for_keyword): New.
	(add_note_about_new_keyword): New.
	* c-parser.cc (report_bad_enum_name): New, split out from...
	(c_parser_enum_specifier): ...here, adding handling for RID_FALSE
	and RID_TRUE.
	* c-tree.h (add_note_about_new_keyword): New decl.

gcc/testsuite/ChangeLog:
	PR c/117629
	* gcc.dg/auto-type-2.c: Update expected output with _Bool.
	* gcc.dg/c23-bool-errors-1.c: New test.
	* gcc.dg/c23-bool-errors-2.c: New test.
	* gcc.dg/c23-bool-errors-3.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

1 files changed, 42 insertions, 10 deletions

diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index d289af6..f72b0f0 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -3785,6 +3785,47 @@ c_parser_declspecs (c_parser *parser, struct c_declspecs *specs,
     specs->postfix_attrs = c_parser_std_attribute_specifier_sequence (parser);
 }
 
+/* Complain about a non-CPP_NAME within an enumerator list.  */
+
+static void
+report_bad_enum_name (c_parser *parser)
+{
+  if (!parser->error)
+    {
+      c_token *token = c_parser_peek_token (parser);
+      switch (token->type)
+	{
+	default:
+	  break;
+	case CPP_CLOSE_BRACE:
+	  /* Give a nicer error for "enum {}".  */
+	  error_at (token->location,
+		    "empty enum is invalid");
+	  parser->error = true;
+	  return;
+	case CPP_KEYWORD:
+	  /* Give a nicer error for attempts to use "true" and "false"
+	     in enums with C23 onwards.  */
+	  if (token->keyword == RID_FALSE
+	      || token->keyword == RID_TRUE)
+	    {
+	      auto_diagnostic_group d;
+	      error_at (token->location,
+			"cannot use keyword %qs as enumeration constant",
+			IDENTIFIER_POINTER (token->value));
+	      add_note_about_new_keyword (token->location,
+					  token->value);
+	      parser->error = true;
+	      return;
+	    }
+	  break;
+	}
+    }
+
+  /* Otherwise, a more generic error message.  */
+  c_parser_error (parser, "expected identifier");
+}
+
 /* Parse an enum specifier (C90 6.5.2.2, C99 6.7.2.2, C11 6.7.2.2).
 
    enum-specifier:
@@ -3952,16 +3993,7 @@ c_parser_enum_specifier (c_parser *parser)
 	  location_t decl_loc, value_loc;
 	  if (c_parser_next_token_is_not (parser, CPP_NAME))
 	    {
-	      /* Give a nicer error for "enum {}".  */
-	      if (c_parser_next_token_is (parser, CPP_CLOSE_BRACE)
-		  && !parser->error)
-		{
-		  error_at (c_parser_peek_token (parser)->location,
-			    "empty enum is invalid");
-		  parser->error = true;
-		}
-	      else
-		c_parser_error (parser, "expected identifier");
+	      report_bad_enum_name (parser);
 	      c_parser_skip_until_found (parser, CPP_CLOSE_BRACE, NULL);
 	      values = error_mark_node;
 	      break;