[1/3] C-family, Objective-C [1/3] : Implement Wobjc-root-class [PR77404].

diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
index 067dfabe0a4..6718fff6efb 100644
--- a/gcc/c-family/c-attribs.c
+++ b/gcc/c-family/c-attribs.c
@@ -160,6 +160,7 @@  static tree handle_patchable_function_entry_attribute  
(tree *, tree, tree,
  						       int, bool *);
  static tree handle_copy_attribute (tree *, tree, tree, int, bool *);
  static tree handle_nsobject_attribute (tree *, tree, tree, int, bool *);
+static tree handle_objc_root_class_attribute (tree *, tree, tree, int,  
bool *);
 
  /* Helper to define attribute exclusions.  */
  #define ATTR_EXCL(name, function, type, variable)	\
@@ -517,6 +518,8 @@  const struct attribute_spec c_common_attribute_table[] =
    /* Attributes used by Objective-C.  */
    { "NSObject",		      0, 0, true, false, false, false,
  			      handle_nsobject_attribute, NULL },
+  { "objc_root_class",	      0, 0, true, false, false, false,
+			      handle_objc_root_class_attribute, NULL },
    { NULL,                     0, 0, false, false, false, false, NULL, NULL }
  };
 
@@ -5232,6 +5235,22 @@  handle_nsobject_attribute (tree *node, tree name,  
tree args,
    return NULL_TREE;
  }
 
+/* Handle a "objc_root_class" attributes; arguments as in
+   struct attribute_spec.handler.  */
+
+static tree
+handle_objc_root_class_attribute (tree */*node*/, tree name, tree /*args*/,
+				  int /*flags*/, bool *no_add_attrs)
+{
+  /* This has no meaning outside Objective-C.  */
+  if (!c_dialect_objc())
+    warning (OPT_Wattributes, "%qE is only applicable to Objective-C"
+	     " class interfaces, attribute ignored", name);
+
+  *no_add_attrs = true;
+  return NULL_TREE;
+}
+
  /* Attempt to partially validate a single attribute ATTR as if
     it were to be applied to an entity OPER.  */
 
diff --git a/gcc/c-family/c-objc.h b/gcc/c-family/c-objc.h
index 6e96731882b..9414aecc668 100644
--- a/gcc/c-family/c-objc.h
+++ b/gcc/c-family/c-objc.h
@@ -124,7 +124,7 @@  extern tree objc_get_protocol_qualified_type (tree,  
tree);
  extern tree objc_get_class_reference (tree);
  extern tree objc_get_class_ivars (tree);
  extern bool objc_detect_field_duplicates (bool);
-extern void objc_start_class_interface (tree, tree, tree, tree);
+extern void objc_start_class_interface (tree, location_t, tree, tree, tree);
  extern void objc_start_category_interface (tree, tree, tree, tree);
  extern void objc_start_protocol (tree, tree, tree);
  extern void objc_continue_interface (void);
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 5fada472db1..1dad951e253 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -1002,6 +1002,11 @@  Enum(cpp_normalize_level) String(id)  
Value(normalized_identifier_C)
  EnumValue
  Enum(cpp_normalize_level) String(nfc) Value(normalized_C)
 
+Wobjc-root-class
+ObjC ObjC++ Var(warn_objc_root_class) Warning Init(1)
+Warn if a class interface has no superclass.  Root classes may use an  
attribute
+to suppress this warning.
+
  Wold-style-cast
  C++ ObjC++ Var(warn_old_style_cast) Warning
  Warn if a C-style cast is used in a program.
diff --git a/gcc/c-family/stub-objc.c b/gcc/c-family/stub-objc.c
index 2f535578789..26941aa2cb4 100644
--- a/gcc/c-family/stub-objc.c
+++ b/gcc/c-family/stub-objc.c
@@ -137,6 +137,7 @@  objc_set_method_opt (bool ARG_UNUSED (optional))
 
  void
  objc_start_class_interface (tree ARG_UNUSED (name),
+			    location_t /*name_loc*/,
  			    tree ARG_UNUSED (super),
  			    tree ARG_UNUSED (protos),
  			    tree ARG_UNUSED (attribs))
diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 377914cad16..f4c4cf7bf8f 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -10801,6 +10801,7 @@  c_parser_objc_class_definition (c_parser *parser,  
tree attributes)
        return;
      }
    id1 = c_parser_peek_token (parser)->value;
+  location_t loc1 = c_parser_peek_token (parser)->location;
    c_parser_consume_token (parser);
    if (c_parser_next_token_is (parser, CPP_OPEN_PAREN))
      {
@@ -10860,7 +10861,7 @@  c_parser_objc_class_definition (c_parser *parser,  
tree attributes)
        tree proto = NULL_TREE;
        if (c_parser_next_token_is (parser, CPP_LESS))
  	proto = c_parser_objc_protocol_refs (parser);
-      objc_start_class_interface (id1, superclass, proto, attributes);
+      objc_start_class_interface (id1, loc1, superclass, proto, attributes);
      }
    else
      objc_start_class_implementation (id1, superclass);
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 6eb9280f056..9d050f72fc1 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -33584,6 +33584,7 @@  cp_parser_objc_class_interface (cp_parser* parser,  
tree attributes)
    bool is_class_extension;
 
    cp_lexer_consume_token (parser->lexer);  /* Eat '@interface'.  */
+  location_t nam_loc = cp_lexer_peek_token (parser->lexer)->location;
    name = cp_parser_identifier (parser);
    if (name == error_mark_node)
      {
@@ -33603,7 +33604,7 @@  cp_parser_objc_class_interface (cp_parser* parser,  
tree attributes)
      objc_start_category_interface (name, categ, protos, attributes);
    else
      {
-      objc_start_class_interface (name, super, protos, attributes);
+      objc_start_class_interface (name, nam_loc, super, protos, attributes);
        /* Handle instance variable declarations, if any.  */
        cp_parser_objc_class_ivars (parser);
        objc_continue_interface ();
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 8c51e6faac8..aaa5faf76c7 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -8558,6 +8558,12 @@  and caught in another, the class must have default  
visibility.
  Otherwise the two shared objects are unable to use the same
  typeinfo node and exception handling will break.
 
+@item objc_root_class @r{(Objective-C and Objective-C++ only)}
+@cindex @code{objc_root_class} type attribute
+This attribute marks a class as being a root class, and thus allows
+the compiler to elide any warnings about a missing superclass and to
+make additional checks for mandatory methods as needed.
+
  @end table
 
  To specify multiple attributes, separate them by commas within the
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2b0a275a409..b3dc2dbdb0c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -277,7 +277,7 @@  Objective-C and Objective-C++ Dialects}.
  -fzero-link @gol
  -gen-decls @gol
  -Wassign-intercept  -Wno-property-assign-default @gol
--Wno-protocol  -Wselector @gol
+-Wno-protocol -Wobjc-root-class -Wselector @gol
  -Wstrict-selector-match @gol
  -Wundeclared-selector}
 
@@ -4357,6 +4357,13 @@  from the superclass.  If you use the  
@option{-Wno-protocol} option, then
  methods inherited from the superclass are considered to be implemented,
  and no warning is issued for them.
 
+@item -Wobjc-root-class @r{(Objective-C and Objective-C++ only)}
+@opindex Wobjc-root-class
+Warn if a class interface lacks a superclass. Most classes will inherit
+from @code{NSObject} (or @code{Object}) for example.  When declaring
+classes intended to be root classes, the warning can be suppressed by
+marking their interfaces with @code{__attribute__((objc_root_class))}.
+
  @item -Wselector @r{(Objective-C and Objective-C++ only)}
  @opindex Wselector
  @opindex Wno-selector
diff --git a/gcc/objc/objc-act.c b/gcc/objc/objc-act.c
index fc247220a62..0cab12eec50 100644
--- a/gcc/objc/objc-act.c
+++ b/gcc/objc/objc-act.c
@@ -571,11 +571,11 @@  lookup_protocol_in_reflist (tree rproto_list, tree  
lproto)
  }
 
  void
-objc_start_class_interface (tree klass, tree super_class,
+objc_start_class_interface (tree klass, location_t name_loc, tree  
super_class,
  			    tree protos, tree attributes)
  {
    if (flag_objc1_only && attributes)
-    error_at (input_location, "class attributes are not available in  
Objective-C 1.0");
+    error_at (name_loc, "class attributes are not available in Objective-C  
1.0");
 
    objc_interface_context
      = objc_ivar_context
@@ -7030,6 +7030,12 @@  start_class (enum tree_code code, tree class_name,  
tree super_name,
  	  CLASS_SUPER_NAME (objc_implementation_context)
  	    = CLASS_SUPER_NAME (implementation_template);
  	}
+
+      if (!CLASS_SUPER_NAME (objc_implementation_context)
+	  && !lookup_attribute ("objc_root_class",
+				TYPE_ATTRIBUTES (implementation_template)))
+	  warning (OPT_Wobjc_root_class, "class %qE defined without"
+		      " specifying a base class", class_name);
        break;
 
      case CLASS_INTERFACE_TYPE:
@@ -7062,6 +7068,8 @@  start_class (enum tree_code code, tree class_name,  
tree super_name,
  		TREE_DEPRECATED (klass) = 1;
  	      else if (is_attribute_p  ("objc_exception", name))
  		CLASS_HAS_EXCEPTION_ATTR (klass) = 1;
+	      else if (is_attribute_p  ("objc_root_class", name))
+		;
  	      else if (is_attribute_p  ("visibility", name))
  		;
  	      else
diff --git a/gcc/testsuite/obj-c++.dg/attributes/root-class-01.mm  
b/gcc/testsuite/obj-c++.dg/attributes/root-class-01.mm
new file mode 100644
index 00000000000..84da94aa1ff
--- /dev/null
+++ b/gcc/testsuite/obj-c++.dg/attributes/root-class-01.mm
@@ -0,0 +1,11 @@ 
+/* Test Wobjc-root-class warning is suppressed by the objc_root_class attr.
+   Note that we don't issue a warning unless the TU contains an  
implementation
+   for the class.  This should compile without warning.  */
+/*  { dg-additional-options "-fsyntax-only " } */
+
+__attribute__((objc_root_class))
+@interface ARootObject
+@end
+
+@implementation ARootObject
+@end
diff --git a/gcc/testsuite/obj-c++.dg/root-class-00.mm  
b/gcc/testsuite/obj-c++.dg/root-class-00.mm
new file mode 100644
index 00000000000..f951b0de8cc
--- /dev/null
+++ b/gcc/testsuite/obj-c++.dg/root-class-00.mm
@@ -0,0 +1,10 @@ 
+/* Test Wobjc-root-class.
+   Note that we don't issue a warning unless the TU contains an  
implementation
+   for the class.  */
+/*  { dg-additional-options "-fsyntax-only " } */
+
+@interface ARootObject
+@end
+
+@implementation ARootObject /* { dg-warning {class 'ARootObject' defined  
without specifying a base class} } */
+@end
diff --git a/gcc/testsuite/objc.dg/attributes/root-class-01.m  
b/gcc/testsuite/objc.dg/attributes/root-class-01.m
new file mode 100644
index 00000000000..84da94aa1ff
--- /dev/null
+++ b/gcc/testsuite/objc.dg/attributes/root-class-01.m
@@ -0,0 +1,11 @@ 
+/* Test Wobjc-root-class warning is suppressed by the objc_root_class attr.
+   Note that we don't issue a warning unless the TU contains an  
implementation
+   for the class.  This should compile without warning.  */
+/*  { dg-additional-options "-fsyntax-only " } */
+
+__attribute__((objc_root_class))
+@interface ARootObject
+@end
+
+@implementation ARootObject
+@end
diff --git a/gcc/testsuite/objc.dg/root-class-00.m  
b/gcc/testsuite/objc.dg/root-class-00.m
new file mode 100644
index 00000000000..1f73f735761
--- /dev/null
+++ b/gcc/testsuite/objc.dg/root-class-00.m
@@ -0,0 +1,10 @@ 
+/* Test Wobjc-root-class.
+   Note that we don't issue a warning unless the TU contains an  
implementation
+   for the class.  */
+/*  { dg-additional-options "-fsyntax-only " } */
+
+@interface ARootObject
+@end
+
+@implementation ARootObject