Change some bad uses of C2x attributes into pedwarns

Index: gcc/c/c-decl.c
===================================================================
--- gcc/c/c-decl.c	(revision 278418)
+++ gcc/c/c-decl.c	(working copy)
@@ -4516,8 +4516,14 @@  void
 c_warn_unused_attributes (tree attrs)
 {
   for (tree t = attrs; t != NULL_TREE; t = TREE_CHAIN (t))
-    warning (OPT_Wattributes, "%qE attribute ignored",
-	     get_attribute_name (t));
+    if (get_attribute_namespace (t) == NULL_TREE)
+      /* The specifications of standard attributes mean this is a
+	 constraint violation.  */
+      pedwarn (input_location, OPT_Wattributes, "%qE attribute ignored",
+	       get_attribute_name (t));
+    else
+      warning (OPT_Wattributes, "%qE attribute ignored",
+	       get_attribute_name (t));
 }
 
 /* Called when a declaration is seen that contains no names to declare.
Index: gcc/c/c-parser.c
===================================================================
--- gcc/c/c-parser.c	(revision 278418)
+++ gcc/c/c-parser.c	(working copy)
@@ -4803,7 +4803,7 @@  c_parser_balanced_token_sequence (c_parser *parser
 */
 
 static tree
-c_parser_std_attribute (c_parser *parser)
+c_parser_std_attribute (c_parser *parser, bool for_tm)
 {
   c_token *token = c_parser_peek_token (parser);
   tree ns, name, attribute;
@@ -4834,39 +4834,53 @@  static tree
   attribute = build_tree_list (build_tree_list (ns, name), NULL_TREE);
 
   /* Parse the arguments, if any.  */
+  const attribute_spec *as = lookup_attribute_spec (TREE_PURPOSE (attribute));
   if (c_parser_next_token_is_not (parser, CPP_OPEN_PAREN))
-    return attribute;
-  location_t open_loc = c_parser_peek_token (parser)->location;
-  matching_parens parens;
-  parens.consume_open (parser);
-  const attribute_spec *as = lookup_attribute_spec (TREE_PURPOSE (attribute));
-  if ((as && as->max_length == 0)
-      /* Special-case the transactional-memory attribute "outer",
-	 which is specially handled but not registered as an
-	 attribute, to avoid allowing arbitrary balanced token
-	 sequences as arguments.  */
-      || is_attribute_p ("outer", name))
+    goto out;
+  {
+    location_t open_loc = c_parser_peek_token (parser)->location;
+    matching_parens parens;
+    parens.consume_open (parser);
+    if ((as && as->max_length == 0)
+	/* Special-case the transactional-memory attribute "outer",
+	   which is specially handled but not registered as an
+	   attribute, to avoid allowing arbitrary balanced token
+	   sequences as arguments.  */
+	|| is_attribute_p ("outer", name))
+      {
+	error_at (open_loc, "%qE attribute does not take any arguments", name);
+	parens.skip_until_found_close (parser);
+	return error_mark_node;
+      }
+    if (as)
+      {
+	bool takes_identifier
+	  = (ns != NULL_TREE
+	     && strcmp (IDENTIFIER_POINTER (ns), "gnu") == 0
+	     && attribute_takes_identifier_p (name));
+	bool require_string
+	  = (ns == NULL_TREE
+	     && strcmp (IDENTIFIER_POINTER (name), "deprecated") == 0);
+	TREE_VALUE (attribute)
+	  = c_parser_attribute_arguments (parser, takes_identifier,
+					  require_string, false);
+      }
+    else
+      c_parser_balanced_token_sequence (parser);
+    parens.require_close (parser);
+  }
+ out:
+  if (ns == NULL_TREE && !for_tm && !as && !is_attribute_p ("nodiscard", name))
     {
-      error_at (open_loc, "%qE attribute does not take any arguments", name);
-      parens.skip_until_found_close (parser);
+      /* An attribute with standard syntax and no namespace specified
+	 is a constraint violation if it is not one of the known
+	 standard attributes (of which nodiscard is the only one
+	 without a handler in GCC).  Diagnose it here with a pedwarn
+	 and then discard it to prevent a duplicate warning later.  */
+      pedwarn (input_location, OPT_Wattributes, "%qE attribute ignored",
+	       name);
       return error_mark_node;
     }
-  if (as)
-    {
-      bool takes_identifier
-	= (ns != NULL_TREE
-	   && strcmp (IDENTIFIER_POINTER (ns), "gnu") == 0
-	   && attribute_takes_identifier_p (name));
-      bool require_string
-	= (ns == NULL_TREE
-	   && strcmp (IDENTIFIER_POINTER (name), "deprecated") == 0);
-      TREE_VALUE (attribute)
-	= c_parser_attribute_arguments (parser, takes_identifier,
-					require_string, false);
-    }
-  else
-    c_parser_balanced_token_sequence (parser);
-  parens.require_close (parser);
   return attribute;
 }
 
@@ -4898,7 +4912,7 @@  c_parser_std_attribute_specifier (c_parser *parser
 	  c_parser_consume_token (parser);
 	  continue;
 	}
-      tree attribute = c_parser_std_attribute (parser);
+      tree attribute = c_parser_std_attribute (parser, for_tm);
       if (attribute != error_mark_node)
 	{
 	  bool duplicate = false;
Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c	(revision 278418)
+++ gcc/c-family/c-common.c	(working copy)
@@ -5702,7 +5702,15 @@  attribute_fallthrough_p (tree attr)
     {
       tree name = get_attribute_name (t);
       if (!is_attribute_p ("fallthrough", name))
-	warning (OPT_Wattributes, "%qE attribute ignored", name);
+	{
+	  if (!c_dialect_cxx () && get_attribute_namespace (t) == NULL_TREE)
+	    /* The specifications of standard attributes in C mean
+	       this is a constraint violation.  */
+	    pedwarn (input_location, OPT_Wattributes, "%qE attribute ignored",
+		     get_attribute_name (t));
+	  else
+	    warning (OPT_Wattributes, "%qE attribute ignored", name);
+	}
     }
   return true;
 }
Index: gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c	(revision 278418)
+++ gcc/testsuite/gcc.dg/c2x-attr-deprecated-2.c	(working copy)
@@ -5,7 +5,7 @@ 
 /* This attribute is not valid in most cases on types other than their
    definitions, or on statements, or as an attribute-declaration.  */
 
-[[deprecated]]; /* { dg-warning "ignored" } */
+[[deprecated]]; /* { dg-error "ignored" } */
 
 int [[deprecated]] var; /* { dg-warning "ignored" } */
 /* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */
@@ -20,6 +20,6 @@  void
 f (void)
 {
   int a;
-  [[deprecated]]; /* { dg-warning "ignored" } */
-  [[deprecated]] a = 1; /* { dg-warning "ignored" } */
+  [[deprecated]]; /* { dg-error "ignored" } */
+  [[deprecated]] a = 1; /* { dg-error "ignored" } */
 }
Index: gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c	(revision 278418)
+++ gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c	(working copy)
@@ -6,7 +6,7 @@ 
 [[__deprecated__, deprecated("message")]] int b; /* { dg-error "can appear at most once" } */
 int c [[deprecated("message"), deprecated]]; /* { dg-error "can appear at most once" } */
 [[deprecated, deprecated]]; /* { dg-error "can appear at most once" } */
-/* { dg-warning "ignored" "ignored" { target *-*-* } .-1 } */
+/* { dg-error "ignored" "ignored" { target *-*-* } .-1 } */
 
 /* Separate attribute lists in the same attribute specifier sequence,
    with the same attribute in them, are OK.  */
Index: gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c	(revision 278418)
+++ gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c	(working copy)
@@ -31,5 +31,5 @@  f (int a)
       b += 5;
       break;
     }
-  [[fallthrough]] return b; /* { dg-warning "ignored" } */
+  [[fallthrough]] return b; /* { dg-error "ignored" } */
 }
Index: gcc/testsuite/gcc.dg/c2x-attr-fallthrough-5.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-fallthrough-5.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/c2x-attr-fallthrough-5.c	(working copy)
@@ -0,0 +1,30 @@ 
+/* Test C2x fallthrough attribute: mixtures with other attributes.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+/* Use of other standard attributes together with "fallthrough" goes
+   through a different path to diagnosing ignored attributes from that
+   used in attribute declarations without "fallthrough".  Verify that
+   such ignored attributes result in a pedwarn (for use in a context
+   not permitted in the constraints for those attributes) in this case
+   as well.  */
+
+int
+f (int a)
+{
+  switch (a)
+    {
+    case 1:
+      a++;
+      [[fallthrough, deprecated]]; /* { dg-error "attribute ignored" } */
+    case 2:
+      a++;
+      [[maybe_unused]] [[fallthrough]]; /* { dg-error "attribute ignored" } */
+    case 3:
+      a++;
+      [[__nodiscard__, fallthrough]]; /* { dg-error "attribute ignored" } */
+    case 4:
+      a++;
+    }
+  return a;
+}
Index: gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-2.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-2.c	(revision 278418)
+++ gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-2.c	(working copy)
@@ -5,7 +5,7 @@ 
 /* This attribute is not valid in most cases on types other than their
    definitions, or on statements, or as an attribute-declaration.  */
 
-[[maybe_unused]]; /* { dg-warning "ignored" } */
+[[maybe_unused]]; /* { dg-error "ignored" } */
 
 int [[maybe_unused]] var; /* { dg-warning "ignored" } */
 /* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */
@@ -20,6 +20,6 @@  void
 f (void)
 {
   int a;
-  [[maybe_unused]]; /* { dg-warning "ignored" } */
-  [[maybe_unused]] a = 1; /* { dg-warning "ignored" } */
+  [[maybe_unused]]; /* { dg-error "ignored" } */
+  [[maybe_unused]] a = 1; /* { dg-error "ignored" } */
 }
Index: gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c	(revision 278418)
+++ gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c	(working copy)
@@ -6,7 +6,7 @@ 
 [[__maybe_unused__, maybe_unused]] int b; /* { dg-error "can appear at most once" } */
 int c [[maybe_unused, maybe_unused]]; /* { dg-error "can appear at most once" } */
 [[maybe_unused, maybe_unused]]; /* { dg-error "can appear at most once" } */
-/* { dg-warning "ignored" "ignored" { target *-*-* } .-1 } */
+/* { dg-error "ignored" "ignored" { target *-*-* } .-1 } */
 
 /* Separate attribute lists in the same attribute specifier sequence,
    with the same attribute in them, are OK.  */
Index: gcc/testsuite/gcc.dg/c2x-attr-syntax-5.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-syntax-5.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/c2x-attr-syntax-5.c	(working copy)
@@ -0,0 +1,56 @@ 
+/* Test C2x attribute syntax.  Test unknown standard attributes
+   diagnosed with a pedwarn.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+[[unknown_attribute]]; /* { dg-error "attribute ignored" } */
+
+[[unknown_attribute]] extern int a; /* { dg-error "attribute ignored" } */
+extern int [[unknown_attribute(123)]] a; /* { dg-error "attribute ignored" } */
+extern int a [[unknown_attribute("")]]; /* { dg-error "attribute ignored" } */
+
+int f () [[unknown_attribute]]; /* { dg-error "attribute ignored" } */
+int f (void) [[unknown_attribute(1)]]; /* { dg-error "attribute ignored" } */
+int g ([[unknown_attribute]] int a); /* { dg-error "attribute ignored" } */
+int g (int [[unknown_attribute]] a); /* { dg-error "attribute ignored" } */
+int g (int a [[unknown_attribute]]); /* { dg-error "attribute ignored" } */
+int g ([[unknown_attribute]] int); /* { dg-error "attribute ignored" } */
+int g (int [[unknown_attribute]]); /* { dg-error "attribute ignored" } */
+int g (int) [[unknown_attribute]]; /* { dg-error "attribute ignored" } */
+
+int *[[unknown_attribute]] p; /* { dg-error "attribute ignored" } */
+int b[3] [[unknown_attribute]]; /* { dg-error "attribute ignored" } */
+
+int h (int () [[unknown_attribute]]); /* { dg-error "attribute ignored" } */
+
+struct [[unknown_attribute]] s; /* { dg-error "attribute ignored" } */
+union [[unknown_attribute]] u; /* { dg-error "attribute ignored" } */
+
+struct [[unknown_attribute]] s2 { int a; }; /* { dg-error "attribute ignored" } */
+union [[unknown_attribute(x)]] u2 { int a; }; /* { dg-error "attribute ignored" } */
+
+struct s3 { [[unknown_attribute]] int a; }; /* { dg-error "attribute ignored" } */
+struct s4 { int [[unknown_attribute]] a; }; /* { dg-error "attribute ignored" } */
+union u3 { [[unknown_attribute]] int a; }; /* { dg-error "attribute ignored" } */
+union u4 { int [[unknown_attribute]] a; }; /* { dg-error "attribute ignored" } */
+
+int z = sizeof (int [[unknown_attribute]]); /* { dg-error "attribute ignored" } */
+
+enum [[unknown_attribute]] { E1 }; /* { dg-error "attribute ignored" } */
+enum { E2 [[unknown_attribute]] }; /* { dg-error "attribute ignored" } */
+enum { E3 [[unknown_attribute]] = 4 }; /* { dg-error "attribute ignored" } */
+
+void
+func (void) [[unknown_attribute]] { /* { dg-error "attribute ignored" } */
+  [[unknown_attribute]] int var; /* { dg-error "attribute ignored" } */
+  [[unknown_attribute]] { } /* { dg-error "attribute ignored" } */
+  [[unknown_attribute(!)]]; /* { dg-error "attribute ignored" } */
+  [[unknown_attribute]] var = 1; /* { dg-error "attribute ignored" } */
+  [[unknown_attribute]] x: var = 2; /* { dg-error "attribute ignored" } */
+  for ([[unknown_attribute]] int zz = 1; zz < 10; zz++) ; /* { dg-error "attribute ignored" } */
+}
+
+/* nodiscard is not yet implemented, but is a standard attribute, so
+   its use is not a constraint violation and should only receive a
+   warning.  */
+[[nodiscard]] int ndfunc (void); /* { dg-warning "attribute directive ignored" } */