* pt.c (tsubst) [TEMPLATE_TYPE_PARM]: Use TEMPLATE_PARM_DESCENDANTS.

From-SVN: r261861

gcc/cp/ChangeLog

 2018-06-21  Jason Merrill  <jason@redhat.com>
 
+	* pt.c (tsubst) [TEMPLATE_TYPE_PARM]: Use TEMPLATE_PARM_DESCENDANTS.
+
 	* name-lookup.c (do_push_to_top_level): Don't allocate
 	current_lang_base.
 	(do_pop_from_top_level): Release current_lang_base.

gcc/cp/pt.c

@@ -14472,6 +14472,15 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 		     && !PLACEHOLDER_TYPE_CONSTRAINTS (r))
 	      /* Break infinite recursion when substituting the constraints
 		 of a constrained placeholder.  */;
+	    else if (TREE_CODE (t) == TEMPLATE_TYPE_PARM
+		     && !PLACEHOLDER_TYPE_CONSTRAINTS (t)
+		     && !CLASS_PLACEHOLDER_TEMPLATE (t)
+		     && (arg = TEMPLATE_TYPE_PARM_INDEX (t),
+			 r = TEMPLATE_PARM_DESCENDANTS (arg))
+		     && (TEMPLATE_PARM_LEVEL (r)
+			 == TEMPLATE_PARM_LEVEL (arg) - levels))
+		/* Cache the simple case of lowering a type parameter.  */
+	      r = TREE_TYPE (r);
 	    else
 	      {
 		r = copy_type (t);