c++: Check attributes on friend declarations [PR99032]

This patch implements [dcl.attr.grammar]/5: "If an attribute-specifier-seq
appertains to a friend declaration ([class.friend]), that declaration shall
be a definition."

This restriction applies to C++11-style attributes as well as GNU
attributes with the exception that we allow GNU attributes that require
a type, such as vector_size to continue accepting code as in attrib63.C.
There are various forms of friend declarations, we have friend
templates, C++11 extended friend declarations, and so on.  In some cases
we already ignore the attribute and warn that it was ignored.  But
certain cases weren't diagnosed, and with this patch we'll give a hard
error.  I tried hard not to emit both a warning and error and I think it
worked out.

Jason provided the cp_parser_decl_specifier_seq hunk to detect using
standard attributes in the middle of decl-specifiers, which is invalid.

Co-authored-by: Jason Merrill <jason@redhat.com>

gcc/cp/ChangeLog:

	PR c++/99032
	* cp-tree.h (any_non_type_attribute_p): Declare.
	* decl.c (grokdeclarator): Diagnose when an attribute appertains to
	a friend declaration that is not a definition.
	* decl2.c (any_non_type_attribute_p): New.
	* parser.c (cp_parser_decl_specifier_seq): Diagnose standard attributes
	in the middle of decl-specifiers.
	(cp_parser_elaborated_type_specifier): Diagnose when an attribute
	appertains to a friend declaration that is not a definition.
	(cp_parser_member_declaration): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/99032
	* g++.dg/cpp0x/friend7.C: New test.
	* g++.dg/cpp0x/gen-attrs-4.C: Add dg-error.
	* g++.dg/cpp0x/gen-attrs-39-1.C: Likewise.
	* g++.dg/cpp0x/gen-attrs-74.C: New test.
	* g++.dg/ext/attrib63.C: New test.

gcc/cp/cp-tree.h

@@ -6763,6 +6763,7 @@ extern tree grokbitfield (const cp_declarator *, cp_decl_specifier_seq *,
 			  tree, tree, tree);
 extern tree splice_template_attributes		(tree *, tree);
 extern bool any_dependent_type_attributes_p	(tree);
+extern bool any_non_type_attribute_p		(tree);
 extern tree cp_reconstruct_complex_type		(tree, tree);
 extern bool attributes_naming_typedef_ok	(tree);
 extern void cplus_decl_attributes		(tree *, tree, int);

gcc/cp/decl.c

@@ -13741,6 +13741,11 @@ grokdeclarator (const cp_declarator *declarator,
 
 	if (friendp)
 	  {
+	    if (attrlist && !funcdef_flag
+		/* Hack to allow attributes like vector_size on a friend.  */
+		&& any_non_type_attribute_p (*attrlist))
+	      error_at (id_loc, "attribute appertains to a friend "
+			"declaration that is not a definition");
 	    /* Friends are treated specially.  */
 	    if (ctype == current_class_type)
 	      ;  /* We already issued a permerror.  */

gcc/cp/decl2.c

@@ -1331,6 +1331,20 @@ any_dependent_type_attributes_p (tree attrs)
   return false;
 }
 
+/* True if ATTRS contains any attribute that does not require a type.  */
+
+bool
+any_non_type_attribute_p (tree attrs)
+{
+  for (tree a = attrs; a; a = TREE_CHAIN (a))
+    {
+      const attribute_spec *as = lookup_attribute_spec (get_attribute_name (a));
+      if (as && !as->type_required)
+	return true;
+    }
+  return false;
+}
+
 /* Return true iff ATTRS are acceptable attributes to be applied in-place
    to a typedef which gives a previously unnamed class or enum a name for
    linkage purposes.  */

gcc/cp/parser.c

@@ -15146,6 +15146,16 @@ cp_parser_decl_specifier_seq (cp_parser* parser,
       if (!found_decl_spec)
 	break;
 
+      if (decl_specs->std_attributes)
+	{
+	  error_at (decl_specs->locations[ds_std_attribute],
+		    "standard attributes in middle of decl-specifiers");
+	  inform (decl_specs->locations[ds_std_attribute],
+		  "standard attributes must precede the decl-specifiers to "
+		  "apply to the declaration, or follow them to apply to "
+		  "the type");
+	}
+
       decl_specs->any_specifiers_p = true;
       /* After we see one decl-specifier, further decl-specifiers are
 	 always optional.  */
@@ -19764,11 +19774,15 @@ cp_parser_elaborated_type_specifier (cp_parser* parser,
 	       && ! processing_explicit_instantiation)
 	warning (OPT_Wattributes,
 		 "attributes ignored on template instantiation");
+      else if (is_friend && attributes)
+	error ("attribute appertains to a friend declaration that is not "
+	       "a definition");
       else if (is_declaration && cp_parser_declares_only_class_p (parser))
 	cplus_decl_attributes (&type, attributes, (int) ATTR_FLAG_TYPE_IN_PLACE);
       else
 	warning (OPT_Wattributes,
-		 "attributes ignored on elaborated-type-specifier that is not a forward declaration");
+		 "attributes ignored on elaborated-type-specifier that is "
+		 "not a forward declaration");
     }
 
   if (tag_type == enum_type)
@@ -26054,6 +26068,13 @@ cp_parser_member_declaration (cp_parser* parser)
 		 error_at (decl_spec_token_start->location,
 			   "friend declaration does not name a class or "
 			   "function");
+	       /* Give an error if an attribute cannot appear here, as per
+		  [dcl.attr.grammar]/5.  But not when declares_class_or_enum:
+		  we ignore attributes in elaborated-type-specifiers.  */
+	       else if (!declares_class_or_enum && decl_specifiers.attributes)
+		 error_at (decl_spec_token_start->location,
+			   "attribute appertains to a friend declaration "
+			   "that is not a definition");
 	       else
 		 make_friend_class (current_class_type, type,
 				    /*complain=*/true);

gcc/testsuite/g++.dg/cpp0x/friend7.C

@@ -0,0 +1,40 @@
+// PR c++/99032
+// { dg-do compile { target c++11 } }
+
+class X { };
+template<typename T1, typename T2>
+void foo (T1, T2);
+
+struct S {
+  [[deprecated]] friend void f(); // { dg-error "attribute appertains" }
+  [[deprecated]] friend void f2() { }
+  __attribute__((deprecated)) friend void f3(); // { dg-error "attribute appertains" }
+  friend void f3 [[deprecated]] (); // { dg-error "attribute appertains" }
+  friend void f4 [[deprecated]] () { }
+  [[deprecated]] friend void; // { dg-error "attribute appertains" }
+  __attribute__((deprecated)) friend int; // { dg-error "attribute appertains" }
+  friend __attribute__((deprecated)) int; // { dg-error "attribute appertains" }
+  friend int __attribute__((deprecated)); // { dg-error "attribute appertains" }
+  [[deprecated]] friend X; // { dg-error "attribute appertains" }
+  [[deprecated]] friend class N; // { dg-warning "attribute ignored" }
+  friend class [[deprecated]] N2; // { dg-error "attribute appertains" }
+  friend class __attribute__((deprecated)) N3; // { dg-error "attribute appertains" }
+  [[deprecated]] friend void foo<>(int, int); // { dg-error "attribute appertains" }
+  [[deprecated]] friend void ::foo(int, int); // { dg-error "attribute appertains" }
+  // { dg-bogus "should have" "PR100339" { xfail *-*-* } .-1 }
+};
+
+template<typename T>
+class node { };
+
+template<typename T>
+struct A {
+  [[deprecated]] friend T; // { dg-error "attribute appertains" }
+  [[deprecated]] friend class node<T>; // { dg-warning "attribute ignored" }
+  template<typename>
+  [[deprecated]] friend class A; // { dg-warning "attribute ignored" }
+  template<typename>
+  [[deprecated]] friend void bar () { }
+  template<typename>
+  [[deprecated]] friend void baz (); // { dg-error "attribute appertains" }
+};

gcc/testsuite/g++.dg/cpp0x/gen-attrs-39-1.C

@@ -2,7 +2,8 @@
 
 int fragile_block(void) {
   typedef 
-  [[gnu::aligned (16)]] // { dg-warning "ignored" }
+  [[gnu::aligned (16)]] // { dg-error "standard attributes in middle of decl-specifiers" }
+// { dg-warning "attribute ignored" "" { target *-*-* } .-1 }
   struct  {
     int i;
   } XmmUint16;

gcc/testsuite/g++.dg/cpp0x/gen-attrs-4.C

@@ -13,7 +13,8 @@ int one_third [[noreturn]] [[gnu::unused]] (void);
 int [[gnu::unused]] one_half(); // { dg-warning "ignored" }
 
 static
-[[noreturn]] // { dg-warning "ignored" }
+[[noreturn]] // { dg-error "standard attributes in middle of decl-specifiers" }
+// { dg-warning "attribute ignored" "" { target *-*-* } .-1 }
 void two [[gnu::unused]] (void) {}
 
 

gcc/testsuite/g++.dg/cpp0x/gen-attrs-74.C

@@ -0,0 +1,10 @@
+// { dg-do compile { target c++11 } }
+// A C++11 attribute cannot appear in the middle of the decl-specifier-seq,
+// only before it (in which case it appertains to the declaration) or at
+// the end (in which case it appertains to the type).
+
+struct S {
+  friend [[deprecated]] void; // { dg-error "standard attributes in middle of decl-specifiers" }
+  friend [[deprecated]] int fn(); // { dg-error "standard attributes in middle of decl-specifiers" }
+  // { dg-warning "attribute ignored" "" { target *-*-* } .-1 }
+};

gcc/testsuite/g++.dg/ext/attrib63.C

@@ -0,0 +1,16 @@
+// { dg-do compile }
+
+#define vector __attribute__((vector_size(16)))
+class A {
+  friend vector float f();
+  __attribute__((deprecated)) friend void f2(); // { dg-error "attribute appertains" }
+  friend __attribute__((deprecated, vector_size(16))) float f3(); // { dg-error "attribute appertains" }
+  friend __attribute__((vector_size(16), deprecated)) float f4(); // { dg-error "attribute appertains" }
+};
+
+vector float vf;
+vector float
+f ()
+{
+  return vf;
+}