diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 30c7470974d4b62ec6c03b2a7dd37f046983a247..b1603859644fe40a989bbfebacb9aa29584847ff 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -12440,10 +12440,15 @@ grokdeclarator (const cp_declarator *declarator,
 	{
 	  if (typedef_decl)
 	    {
-	      pedwarn (loc, OPT_Wpedantic, "%qs specified with %qT",
-		       key, type);
+	      pedwarn (loc, OPT_Wpedantic, "%qs specified with %qD",
+		       key, typedef_decl);
 	      ok = !flag_pedantic_errors;
-	      type = DECL_ORIGINAL_TYPE (typedef_decl);
+	      if (is_typedef_decl (typedef_decl))
+		type = DECL_ORIGINAL_TYPE (typedef_decl);
+	      else
+		/* PR108099: __int128_t comes from c_common_nodes_and_builtins,
+		   and is not built as a typedef.  */
+		type = TREE_TYPE (typedef_decl);
 	      typedef_decl = NULL_TREE;
 	    }
 	  else if (declspecs->decltype_p)
diff --git a/gcc/testsuite/g++.dg/ext/int128-7.C b/gcc/testsuite/g++.dg/ext/int128-7.C
new file mode 100644
index 0000000000000000000000000000000000000000..bf5e8c40a4b25feacfe7fe12249b07574b64da32
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/int128-7.C
@@ -0,0 +1,4 @@
+// PR c++/108099
+// { dg-do compile { target { c++11 && int128 } } }
+
+using i128 = signed __int128_t;	// { dg-error "specified with" }