c++: Fix decltype of empty pack expansion of parm.

In unevaluated context, we only substitute a single PARM_DECL, not the
entire chain, but the handling of an empty pack expansion was missing that
check.

	PR c++/93140
	* pt.c (tsubst_decl) [PARM_DECL]: Check cp_unevaluated_operand in
	handling of TREE_CHAIN for empty pack.

gcc/cp/ChangeLog

+2020-02-05  Jason Merrill  <jason@redhat.com>
+
+	PR c++/93140
+	* pt.c (tsubst_decl) [PARM_DECL]: Check cp_unevaluated_operand in
+	handling of TREE_CHAIN for empty pack.
+
 2020-02-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/93557

gcc/cp/pt.c

@@ -14076,7 +14076,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 
                 /* Zero-length parameter packs are boring. Just substitute
                    into the chain.  */
-                if (len == 0)
+		if (len == 0 && !cp_unevaluated_operand)
                   RETURN (tsubst (TREE_CHAIN (t), args, complain,
 				  TREE_CHAIN (t)));
               }

gcc/testsuite/g++.dg/cpp0x/variadic-parm1.C

+// PR c++/93140
+// { dg-do compile { target c++11 } }
+
+int
+bar ()
+{
+  return 42;
+}
+
+template <typename... R>
+void foo (R... r, decltype (bar (r...)) x = 0) {}
+
+int
+main ()
+{
+  foo (3);
+}