--- gcc/tree.cc.jj	2024-08-30 16:41:34.712367197 +0200
+++ gcc/tree.cc	2024-09-04 18:52:37.792157965 +0200
@@ -6085,6 +6085,10 @@ type_hash_canon_hash (tree type)
       hstate.add_poly_int (TYPE_VECTOR_SUBPARTS (type));
       break;
 
+    case REFERENCE_TYPE:
+      hstate.add_flag (TYPE_REF_IS_RVALUE (type));
+      break;
+
     default:
       break;
     }
@@ -6127,7 +6131,6 @@ type_cache_hasher::equal (type_hash *a,
     case OPAQUE_TYPE:
     case COMPLEX_TYPE:
     case POINTER_TYPE:
-    case REFERENCE_TYPE:
     case NULLPTR_TYPE:
       return true;
 
@@ -6217,6 +6220,9 @@ type_cache_hasher::equal (type_hash *a,
 	break;
       return false;
 
+    case REFERENCE_TYPE:
+      return TYPE_REF_IS_RVALUE (a->type) == TYPE_REF_IS_RVALUE (b->type);
+
     default:
       return false;
     }
--- gcc/cp/tree.cc.jj	2024-09-04 12:36:51.904244104 +0200
+++ gcc/cp/tree.cc	2024-09-04 17:56:08.946371353 +0200
@@ -5106,6 +5106,27 @@ handle_std_deprecated_attribute (tree *n
   return ret;
 }
 
+/* The C++17 [[maybe_unused]] attribute mostly maps to the GNU unused
+   attribute.  */
+
+static tree
+handle_maybe_unused_attribute (tree *node, tree name, tree args, int flags,
+			       bool *no_add_attrs)
+{
+  tree t = *node;
+  tree ret = handle_unused_attribute (node, name, args, flags, no_add_attrs);
+  if (TYPE_P (*node) && t != *node)
+    pedwarn (input_location, OPT_Wattributes,
+	     "%qE on a type other than class or enumeration definition", name);
+  else if (TREE_CODE (*node) == FIELD_DECL && DECL_UNNAMED_BIT_FIELD (*node))
+    pedwarn (input_location, OPT_Wattributes, "%qE on unnamed bit-field",
+	     name);
+  else if (TREE_CODE (*node) == LABEL_DECL && DECL_NAME (*node) == NULL_TREE)
+    pedwarn (input_location, OPT_Wattributes,
+	     "%qE on %<case%> or %<default%> label", name);
+  return ret;
+}
+
 /* Table of valid C++ attributes.  */
 static const attribute_spec cxx_gnu_attributes[] =
 {
@@ -5132,7 +5153,7 @@ static const attribute_spec std_attribut
   { "deprecated", 0, 1, false, false, false, false,
     handle_std_deprecated_attribute, NULL },
   { "maybe_unused", 0, 0, false, false, false, false,
-    handle_unused_attribute, NULL },
+    handle_maybe_unused_attribute, NULL },
   { "nodiscard", 0, 1, false, false, false, false,
     handle_nodiscard_attribute, NULL },
   { "no_unique_address", 0, 0, true, false, false, false,
--- gcc/testsuite/g++.dg/cpp0x/attr-maybe_unused1.C.jj	2024-09-04 17:28:44.552574792 +0200
+++ gcc/testsuite/g++.dg/cpp0x/attr-maybe_unused1.C	2024-09-04 18:54:14.207069672 +0200
@@ -0,0 +1,148 @@
+// C++ 26 P2552R3 - On the ignorability of standard attributes
+// { dg-do compile { target c++11 } }
+
+int arr[2];
+struct S { int a, b; };
+S arr2[2];
+
+void
+foo (int n)
+{
+  [[maybe_unused]] int x1;
+  [[maybe_unused ("foobar")]] int x2;		// { dg-error "'maybe_unused' attribute does not take any arguments" }
+						// { dg-error "expected primary-expression before 'int'" "" { target *-*-* } .-1 }
+  [[maybe_unused (0)]] int x3;			// { dg-error "'maybe_unused' attribute does not take any arguments" }
+						// { dg-error "expected primary-expression before 'int'" "" { target *-*-* } .-1 }
+
+  auto a = [] [[maybe_unused]] () {};
+  auto b = [] constexpr [[maybe_unused]] {};	// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+						// { dg-error "parameter declaration before lambda declaration specifiers only optional with" "" { target c++20_down } .-1 }
+						// { dg-error "'constexpr' lambda only available with" "" { target c++14_down } .-2 }
+  auto c = [] noexcept [[maybe_unused]] {};	// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+						// { dg-error "parameter declaration before lambda exception specification only optional with" "" { target c++20_down } .-1 }
+  auto d = [] () [[maybe_unused]] {};		// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+  auto e = new int [n] [[maybe_unused]];	// { dg-warning "attributes ignored on outermost array type in new expression" }
+  auto e2 = new int [n] [[maybe_unused]] [42];	// { dg-warning "attributes ignored on outermost array type in new expression" }
+  auto f = new int [n][42] [[maybe_unused]];	// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+  [[maybe_unused]];				// { dg-warning "attributes at the beginning of statement are ignored" }
+  [[maybe_unused]] {}				// { dg-warning "attributes at the beginning of statement are ignored" }
+  [[maybe_unused]] if (true) {}			// { dg-warning "attributes at the beginning of statement are ignored" }
+  [[maybe_unused]] while (false) {}		// { dg-warning "attributes at the beginning of statement are ignored" }
+  [[maybe_unused]] goto lab;			// { dg-warning "attributes at the beginning of statement are ignored" }
+  [[maybe_unused]] lab:;
+  [[maybe_unused]] try {} catch (int) {}	// { dg-warning "attributes at the beginning of statement are ignored" }
+  if ([[maybe_unused]] int x = 0) {}
+  switch (n)
+    {
+    [[maybe_unused]] case 1:			// { dg-error "'maybe_unused' on 'case' or 'default' label" }
+    [[maybe_unused]] break;			// { dg-warning "attributes at the beginning of statement are ignored" }
+    [[maybe_unused]] default:			// { dg-error "'maybe_unused' on 'case' or 'default' label" }
+	 break;
+    }
+  for ([[maybe_unused]] auto a : arr) {}
+  for ([[maybe_unused]] auto [a, b] : arr2) {}	// { dg-error "structured bindings only available with" "" { target c++14_down } }
+  [[maybe_unused]] asm ("");			// { dg-warning "attributes ignored on 'asm' declaration" }
+  try {} catch ([[maybe_unused]] int x) {}
+  try {} catch ([[maybe_unused]] int) {}
+  try {} catch (int [[maybe_unused]] x) {}	// { dg-warning "attribute ignored" }
+  try {} catch (int [[maybe_unused]]) {}	// { dg-warning "attribute ignored" }
+  try {} catch (int x [[maybe_unused]]) {}
+}
+
+[[maybe_unused]] int bar ();
+using foobar [[maybe_unused]] = int;
+[[maybe_unused]] int a;
+[[maybe_unused]] auto [b, c] = arr;		// { dg-error "structured bindings only available with" "" { target c++14_down } }
+[[maybe_unused]];				// { dg-warning "attribute ignored" }
+inline [[maybe_unused]] void baz () {}		// { dg-warning "attribute ignored" }
+						// { dg-error "standard attributes in middle of decl-specifiers" "" { target *-*-* } .-1 }
+constexpr [[maybe_unused]] int qux () { return 0; }	// { dg-warning "attribute ignored" }
+						// { dg-error "standard attributes in middle of decl-specifiers" "" { target *-*-* } .-1 }
+int [[maybe_unused]] d;				// { dg-warning "attribute ignored" }
+int const [[maybe_unused]] e = 1;		// { dg-warning "attribute ignored" }
+struct A {} [[maybe_unused]];			// { dg-warning "attribute ignored in declaration of 'struct A'" }
+struct A [[maybe_unused]];			// { dg-warning "attribute ignored" }
+struct A [[maybe_unused]] a1;			// { dg-warning "attribute ignored" }
+A [[maybe_unused]] a2;				// { dg-warning "attribute ignored" }
+enum B { B0 } [[maybe_unused]];			// { dg-warning "attribute ignored in declaration of 'enum B'" }
+enum B [[maybe_unused]];			// { dg-warning "attribute ignored" }
+enum B [[maybe_unused]] b1;			// { dg-warning "attribute ignored" }
+B [[maybe_unused]] b2;				// { dg-warning "attribute ignored" }
+struct [[maybe_unused]] C {};
+int f [[maybe_unused]];
+int g[2] [[maybe_unused]];			// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+int g2 [[maybe_unused]] [2];
+int corge () [[maybe_unused]];			// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+int *[[maybe_unused]] h;			// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+int & [[maybe_unused]] i = f;			// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+int && [[maybe_unused]] j = 0;			// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+int S::* [[maybe_unused]] k;			// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+auto l = sizeof (int [2] [[maybe_unused]]);	// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+int freddy ([[maybe_unused]] int a,
+	    [[maybe_unused]] int,
+	    [[maybe_unused]] int c = 0,
+	    [[maybe_unused]] int = 0);
+void
+corge ([[maybe_unused]] int a,
+       [[maybe_unused]] int,
+       [[maybe_unused]] int c = 0,
+       [[maybe_unused]] int = 0)
+{
+}
+[[maybe_unused]] void
+garply ()
+{
+}
+int grault (int [[maybe_unused]] a,		// { dg-warning "attribute ignored" }
+	    int [[maybe_unused]],		// { dg-warning "attribute ignored" }
+	    int [[maybe_unused]] c = 0,		// { dg-warning "attribute ignored" }
+	    int [[maybe_unused]] = 0);		// { dg-warning "attribute ignored" }
+void
+waldo (int [[maybe_unused]] a,			// { dg-warning "attribute ignored" }
+       int [[maybe_unused]],			// { dg-warning "attribute ignored" }
+       int [[maybe_unused]] c = 0,		// { dg-warning "attribute ignored" }
+       int [[maybe_unused]] = 0)		// { dg-warning "attribute ignored" }
+{
+}
+int plugh (int a [[maybe_unused]],
+	    int b [[maybe_unused]] = 0);
+void
+thud (int a [[maybe_unused]],
+      int b [[maybe_unused]] = 0)
+{
+}
+enum [[maybe_unused]] D { D0 };
+enum class [[maybe_unused]] E { E0 };
+enum F {};
+enum [[maybe_unused]] F;			// { dg-warning "type attributes ignored after type is already defined" }
+enum G {
+  G0 [[maybe_unused]],
+  G1 [[maybe_unused]] = 2
+};
+namespace [[maybe_unused]] H { using H0 = int; }// { dg-warning "'maybe_unused' attribute directive ignored" }
+namespace [[maybe_unused]] {}			// { dg-warning "'maybe_unused' attribute directive ignored" }
+[[maybe_unused]] using namespace H;		// { dg-warning "'maybe_unused' attribute directive ignored" }
+struct [[maybe_unused]] I
+{
+  [[maybe_unused]];				// { dg-error "declaration does not declare anything" }
+  [[maybe_unused]] int i;
+  [[maybe_unused]] int foo ();
+  [[maybe_unused]] int bar () { return 1; }
+  [[maybe_unused]] int : 0;			// { dg-error "'maybe_unused' on unnamed bit-field" }
+  [[maybe_unused]] int i2 : 5;
+  [[maybe_unused]] static int i3;
+  static int i4;
+};
+[[maybe_unused]] int I::i4 = 0;
+struct J : [[maybe_unused]] C {};		// { dg-warning "attributes on base specifiers are ignored" }
+#if __cpp_concepts >= 201907L
+template <typename T>
+concept K [[maybe_unused]] = requires { true; };// { dg-warning "'maybe_unused' attribute ignored" "" { target c++20 } }
+#endif
+typedef int L [[maybe_unused]];
+template <typename T>
+struct M {};
+template <>
+struct [[maybe_unused]] M<int> { int m; };
+typedef int N[2] [[maybe_unused]];		// { dg-error "'maybe_unused' on a type other than class or enumeration definition" }
+typedef int O [[maybe_unused]] [2];
