c++: DR 1914 - Allow duplicate standard attributes.

Following Joseph's change for C to allow duplicate C2x standard attributes
<https://gcc.gnu.org/pipermail/gcc-patches/2020-October/557272.html>,
this patch does a similar thing for C++.  This is DR 1914, to be resolved by
<wg21.link/p2156>, which is not part of the standard yet, but has wide
support so looks like a shoo-in.  The duplications now produce warnings
instead, but only if the attribute wasn't specified via a macro.

gcc/c-family/ChangeLog:

	DR 1914
	* c-common.c (attribute_fallthrough_p): Tweak the warning
	message.

gcc/cp/ChangeLog:

	DR 1914
	* parser.c (cp_parser_check_std_attribute): Return bool.  Add a
	location_t parameter.  Return true if the attribute wasn't duplicated.
	Give a warning instead of an error.  Check more attributes.
	(cp_parser_std_attribute_list): Don't add duplicated attributes to
	the list.  Pass location to cp_parser_check_std_attribute.

gcc/testsuite/ChangeLog:

	DR 1914
	* c-c++-common/attr-fallthrough-2.c: Adjust dg-warning.
	* g++.dg/cpp0x/fallthrough2.C: Likewise.
	* g++.dg/cpp0x/gen-attrs-60.C: Turn dg-error into dg-warning.
	* g++.dg/cpp1y/attr-deprecated-2.C: Likewise.
	* g++.dg/cpp2a/attr-likely2.C: Adjust dg-warning.
	* g++.dg/cpp2a/nodiscard-once.C: Turn dg-error into dg-warning.
	* g++.dg/cpp0x/gen-attrs-72.C: New test.

gcc/c-family/c-common.c

@@ -5752,9 +5752,10 @@ attribute_fallthrough_p (tree attr)
   tree t = lookup_attribute ("fallthrough", attr);
   if (t == NULL_TREE)
     return false;
-  /* This attribute shall appear at most once in each attribute-list.  */
+  /* It is no longer true that "this attribute shall appear at most once in
+     each attribute-list", but we still give a warning.  */
   if (lookup_attribute ("fallthrough", TREE_CHAIN (t)))
-    warning (OPT_Wattributes, "%<fallthrough%> attribute specified multiple "
+    warning (OPT_Wattributes, "attribute %<fallthrough%> specified multiple "
 	     "times");
   /* No attribute-argument-clause shall be present.  */
   else if (TREE_VALUE (t) != NULL_TREE)

gcc/cp/parser.c

@@ -27280,30 +27280,30 @@ cp_parser_std_attribute (cp_parser *parser, tree attr_ns)
   return attribute;
 }
 
-/* Check that the attribute ATTRIBUTE appears at most once in the
-   attribute-list ATTRIBUTES.  This is enforced for noreturn (7.6.3),
-   nodiscard, and deprecated (7.6.5).  Note that
-   carries_dependency (7.6.4) isn't implemented yet in GCC.  */
+/* Warn if the attribute ATTRIBUTE appears more than once in the
+   attribute-list ATTRIBUTES.  This used to be enforced for certain
+   attributes, but the restriction was removed in P2156.  Note that
+   carries_dependency ([dcl.attr.depend]) isn't implemented yet in GCC.
+   LOC is the location of ATTRIBUTE.  Returns true if ATTRIBUTE was not
+   found in ATTRIBUTES.  */
 
-static void
-cp_parser_check_std_attribute (tree attributes, tree attribute)
+static bool
+cp_parser_check_std_attribute (location_t loc, tree attributes, tree attribute)
 {
+  static auto alist = { "noreturn", "deprecated", "nodiscard", "maybe_unused",
+			"likely", "unlikely", "fallthrough",
+			"no_unique_address" };
   if (attributes)
-    {
-      tree name = get_attribute_name (attribute);
-      if (is_attribute_p ("noreturn", name)
-	  && lookup_attribute ("noreturn", attributes))
-	error ("attribute %<noreturn%> can appear at most once "
-	       "in an attribute-list");
-      else if (is_attribute_p ("deprecated", name)
-	       && lookup_attribute ("deprecated", attributes))
-	error ("attribute %<deprecated%> can appear at most once "
-	       "in an attribute-list");
-      else if (is_attribute_p ("nodiscard", name)
-	       && lookup_attribute ("nodiscard", attributes))
-	error ("attribute %<nodiscard%> can appear at most once "
-	       "in an attribute-list");
-    }
+    for (const auto &a : alist)
+      if (is_attribute_p (a, get_attribute_name (attribute))
+	  && lookup_attribute (a, attributes))
+	{
+	  if (!from_macro_expansion_at (loc))
+	    warning_at (loc, OPT_Wattributes, "attribute %qs specified "
+			"multiple times", a);
+	  return false;
+	}
+  return true;
 }
 
 /* Parse a list of standard C++-11 attributes.
@@ -27323,14 +27323,17 @@ cp_parser_std_attribute_list (cp_parser *parser, tree attr_ns)
 
   while (true)
     {
+      location_t loc = cp_lexer_peek_token (parser->lexer)->location;
       attribute = cp_parser_std_attribute (parser, attr_ns);
       if (attribute == error_mark_node)
 	break;
       if (attribute != NULL_TREE)
 	{
-	  cp_parser_check_std_attribute (attributes, attribute);
-	  TREE_CHAIN (attribute) = attributes;
-	  attributes = attribute;
+	  if (cp_parser_check_std_attribute (loc, attributes, attribute))
+	    {
+	      TREE_CHAIN (attribute) = attributes;
+	      attributes = attribute;
+	    }
 	}
       token = cp_lexer_peek_token (parser->lexer);
       if (token->type == CPP_ELLIPSIS)

gcc/testsuite/c-c++-common/attr-fallthrough-2.c

@@ -34,7 +34,7 @@ fn (int i)
     __attribute__((fallthrough ("x"))); /* { dg-warning "specified with a parameter" } */
   case 7:
     bar (1);
-    __attribute__((fallthrough, fallthrough)); /* { dg-warning "attribute specified multiple times" } */
+    __attribute__((fallthrough, fallthrough)); /* { dg-warning "specified multiple times" } */
   case 8:
     bar (1);
     __attribute__((fallthrough));

gcc/testsuite/g++.dg/cpp0x/fallthrough2.C

@@ -14,7 +14,7 @@ f (int i)
       [[fallthrough]];
     case 3:
       bar (1);
-      [[gnu::fallthrough, gnu::fallthrough]]; // { dg-warning ".fallthrough. attribute specified multiple times" }
+      [[gnu::fallthrough, gnu::fallthrough]]; // { dg-warning ".fallthrough. specified multiple times" }
     case 2:
       bar (2);
     }

gcc/testsuite/g++.dg/cpp0x/gen-attrs-60.C

@@ -1,4 +1,4 @@
 // PR c++/60365
 // { dg-do compile { target c++11 } }
 
-void func [[noreturn, noreturn]] ();     // { dg-error "at most once" }
+void func [[noreturn, noreturn]] (); // { dg-warning "specified multiple times" }

gcc/testsuite/g++.dg/cpp0x/gen-attrs-72.C

@@ -0,0 +1,45 @@
+// DR 1914 - Duplicate standard attributes 
+// { dg-do compile { target c++11 } }
+
+#define ATTR_NORETURN [[noreturn, noreturn]] 
+
+[[noreturn, noreturn]] void fn0(); // { dg-warning "specified multiple times" }
+ATTR_NORETURN void fn0a();
+[[noreturn]] [[noreturn]] void fn1();
+[[deprecated, deprecated]] void fn2(); // { dg-warning "specified multiple times" }
+[[deprecated]] [[deprecated]] void fn3();
+[[maybe_unused]] [[maybe_unused]] int fn4();
+[[maybe_unused, maybe_unused]] int fn5(); // { dg-warning "specified multiple times" }
+[[nodiscard]] [[nodiscard]] int fn6();
+[[nodiscard, nodiscard]] int fn7(); // { dg-warning "specified multiple times" }
+
+struct E { };
+struct A {
+  [[no_unique_address]] [[no_unique_address]] E e;
+};
+struct B {
+  [[no_unique_address, no_unique_address]] E e; // { dg-warning "specified multiple times" }
+};
+
+int
+f (int n)
+{
+  switch (n)
+    {
+    case 1:
+      [[fallthrough, fallthrough]]; // { dg-warning "specified multiple times" }
+    case 2:
+      [[fallthrough]] [[fallthrough]]; // { dg-warning "specified multiple times" }
+    case 3:
+      return 15;
+    }
+
+  if (n == 10)
+    [[likely]] [[likely]] return 42; // { dg-warning "ignoring attribute" }
+  else if (n == 11)
+    [[unlikely]] [[unlikely]] return 10; // { dg-warning "ignoring attribute" }
+  else if (n == 12)
+    [[likely, likely]] return 42; // { dg-warning "specified multiple times" }
+  else
+    [[unlikely, unlikely]] return 0; // { dg-warning "specified multiple times" }
+}

gcc/testsuite/g++.dg/cpp1y/attr-deprecated-2.C

@@ -1,4 +1,4 @@
 // PR c++/60365
 // { dg-do compile { target c++14 } }
 
-void func [[deprecated, deprecated]] (); // { dg-error "at most once" }
+void func [[deprecated, deprecated]] (); // { dg-warning "specified multiple times" }

gcc/testsuite/g++.dg/cpp2a/attr-likely2.C

@@ -4,7 +4,7 @@ bool b;
 int main()
 {
   if (b)
-    [[likely, likely]] b;	// { dg-warning "ignoring" }
+    [[likely, likely]] b;	// { dg-warning "specified multiple times" }
   else
     [[unlikely]] [[likely]] b;	// { dg-warning "ignoring" }
 

gcc/testsuite/g++.dg/cpp2a/nodiscard-once.C

@@ -2,7 +2,7 @@
 /* { dg-do compile { target c++20 } } */
 /* { dg-options "-O -ftrack-macro-expansion=0" } */
 
-[[nodiscard, nodiscard]] int check1 (void); /* { dg-error "nodiscard\[^\n\r]*can appear at most once" } */
+[[nodiscard, nodiscard]] int check1 (void); // { dg-warning "specified multiple times" }
 
 void
 test (void)