c++: pack indexing and if consteval

The pack index is manifestly constant-evaluated, and the call to
maybe_constant_value needs to reflect that or we wrongly complain about
non-constant index if the evaluation uses if consteval.

gcc/cp/ChangeLog:

	* semantics.cc (finish_type_pack_element): Pass mce_true to
	maybe_constant_value.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/pack-indexing16.C: New test.

gcc/cp/semantics.cc

@@ -5086,7 +5086,7 @@ finish_underlying_type (tree type)
 static tree
 finish_type_pack_element (tree idx, tree types, tsubst_flags_t complain)
 {
-  idx = maybe_constant_value (idx);
+  idx = maybe_constant_value (idx, NULL_TREE, mce_true);
   if (TREE_CODE (idx) != INTEGER_CST || !INTEGRAL_TYPE_P (TREE_TYPE (idx)))
     {
       if (complain & tf_error)

gcc/testsuite/g++.dg/cpp26/pack-indexing16.C

@@ -0,0 +1,16 @@
+// { dg-do compile { target c++26 } }
+
+int i;
+constexpr int idx()
+{
+  if consteval { return 0; }
+  else { return i; }
+}
+
+template <int... Ns>
+int first () { return Ns...[idx()]; }
+
+int main()
+{
+  return first<0,1,2>();
+}