--- gcc/cp/mangle.cc.jj	2026-04-04 11:17:15.921287840 +0200
+++ gcc/cp/mangle.cc	2026-04-06 09:14:12.484890203 +0200
@@ -2745,16 +2745,19 @@ write_type (tree type)
 
 	    case PACK_INDEX_TYPE:
 	      /* https://github.com/itanium-cxx-abi/cxx-abi/issues/175.  */
+	      write_string ("Dy");
 	      if (TREE_CODE (PACK_INDEX_PACK (type)) == TREE_VEC)
 		{
-		  /* TODO: How should this be mangled when the pack is already
-		     expanded?  */
-		  sorry ("mangling type pack index");
-		  break;
+		  write_char ('J');
+		  for (int i = 0; i < TREE_VEC_LENGTH (PACK_INDEX_PACK (type));
+		       ++i)
+		    write_template_arg (TREE_VEC_ELT (PACK_INDEX_PACK (type),
+						      i));
+		  write_char ('E');
 		}
-	      write_string ("Dy");
-	      /* Dy rather than DyDp.  */
-	      write_type (PACK_EXPANSION_PATTERN (PACK_INDEX_PACK (type)));
+	      else
+		/* Dy rather than DyDp.  */
+		write_type (PACK_EXPANSION_PATTERN (PACK_INDEX_PACK (type)));
 	      write_expression (PACK_INDEX_INDEX (type));
 	      break;
 
@@ -3614,17 +3617,19 @@ write_expression (tree expr)
   else if (code == PACK_INDEX_EXPR)
     {
       /* https://github.com/itanium-cxx-abi/cxx-abi/issues/175.  */
+      write_string ("sy");
       if (TREE_CODE (PACK_INDEX_PACK (expr)) == TREE_VEC)
-	/* TODO: How should this be mangled when the pack is already
-	   expanded?  */
-	sorry ("mangling type pack index");
-      else
 	{
-	  write_string ("sy");
-	  /* sy rather than sysp.  */
-	  write_expression (PACK_EXPANSION_PATTERN (PACK_INDEX_PACK (expr)));
-	  write_expression (PACK_INDEX_INDEX (expr));
+	  write_char ('J');
+	  for (int i = 0; i < TREE_VEC_LENGTH (PACK_INDEX_PACK (expr));
+	       ++i)
+	    write_template_arg (TREE_VEC_ELT (PACK_INDEX_PACK (expr), i));
+	  write_char ('E');
 	}
+      else
+	/* sy rather than sysp.  */
+	write_expression (PACK_EXPANSION_PATTERN (PACK_INDEX_PACK (expr)));
+      write_expression (PACK_INDEX_INDEX (expr));
     }
   else if (TREE_CODE (expr) == ALIGNOF_EXPR)
     {
--- gcc/testsuite/g++.dg/cpp26/pack-indexing9.C.jj	2026-03-27 10:17:15.656305903 +0100
+++ gcc/testsuite/g++.dg/cpp26/pack-indexing9.C	2026-04-06 09:27:43.438279320 +0200
@@ -4,24 +4,42 @@
 // From <https://github.com/itanium-cxx-abi/cxx-abi/issues/175>.
 
 template <class... T> struct tuple {
-  template <unsigned I> T...[I] get();  // { dg-bogus "sorry, unimplemented: mangling" "" { xfail *-*-* } }
+  template <unsigned I> T...[I] get();
 };
 
 int
 g ()
 {
   tuple<int> t;
+  // { dg-final { scan-assembler "_ZN5tupleIJiEE3getILj0EEEDyJiET_v" } }
   return t.get<0>();
 }
 
 template<typename T, typename U> concept C = true;
+template<typename T, auto U> concept D = true;
 template<typename ...T> struct A {
-    template<int I, typename ...U> void f(T...[I], U...[I]) requires C<T...[I], U...[I]>;  // { dg-message "sorry, unimplemented: mangling" }
+  template<int I, typename ...U> void f(T...[I], U...[I]) requires C<T...[I], U...[I]>;
+  template<int I, auto ...U> void h(T...[I], decltype(U...[I])) requires D<T...[I], U...[I]>;
 };
 
 void
 h ()
 {
   A<char, int, double> a;
+  // { dg-final { scan-assembler "_ZN1AIJcidEE1fILi1EJiicEEEvDyJcidET_DyT0_T_Q1CIDyT_TL0__DyTL0_0_TL0__E" } }
   a.f<1, int, int, char>(1, 2);
+  // { dg-final { scan-assembler "_ZN1AIJcidEE1hILi1EJLi2ELl3ELj4EEEEvDyJcidET_DtsyT0_T_EQ1DIDyT_TL0__XsyTL0_0_TL0__EE" } }
+  a.h<1, 2, 3L, 4U>(1, 3L);
+}
+
+template <auto... U> struct E {
+  template <unsigned I> decltype (U...[I]) get();
+};
+
+int
+i ()
+{
+  E<1, 2L, 3U, 4LL> t;
+  // { dg-final { scan-assembler "_ZN1EIJLi1ELl2ELj3ELx4EEE3getILj2EEEDtsyJLi1ELl2ELj3ELx4EET_Ev" } }
+  return t.get<2>();
 }