re PR c++/63628 ([c++1y] cannot use decltype on captured arg-pack)

	PR c++/63628
	* pt.c (tsubst_pack_expansion): Also make dummy decls if
	retrieve_local_specialization fails.

From-SVN: r231713

gcc/cp/ChangeLog

+2015-12-16  Jason Merrill  <jason@redhat.com>
+
+	PR c++/63628
+	* pt.c (tsubst_pack_expansion): Also make dummy decls if
+	retrieve_local_specialization fails.
+
 2015-12-16  David Malcolm  <dmalcolm@redhat.com>
 
 	* parser.c (cp_lexer_peek_conflict_marker): New function.

gcc/cp/pt.c

@@ -10803,12 +10803,16 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	  if (PACK_EXPANSION_LOCAL_P (t) || CONSTRAINT_VAR_P (parm_pack))
 	    arg_pack = retrieve_local_specialization (parm_pack);
 	  else
+	    /* We can't rely on local_specializations for a parameter
+	       name used later in a function declaration (such as in a
+	       late-specified return type).  Even if it exists, it might
+	       have the wrong value for a recursive call.  */
+	    need_local_specializations = true;
+
+	  if (!arg_pack)
 	    {
-	      /* We can't rely on local_specializations for a parameter
-		 name used later in a function declaration (such as in a
-		 late-specified return type).  Even if it exists, it might
-		 have the wrong value for a recursive call.  Just make a
-		 dummy decl, since it's only used for its type.  */
+	      /* This parameter pack was used in an unevaluated context.  Just
+		 make a dummy decl, since it's only used for its type.  */
 	      arg_pack = tsubst_decl (parm_pack, args, complain);
 	      if (arg_pack && DECL_PACK_P (arg_pack))
 		/* Partial instantiation of the parm_pack, we can't build
@@ -10816,7 +10820,6 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 		arg_pack = NULL_TREE;
 	      else
 		arg_pack = make_fnparm_pack (arg_pack);
-	      need_local_specializations = true;
 	    }
 	}
       else if (TREE_CODE (parm_pack) == FIELD_DECL)

gcc/testsuite/g++.dg/cpp1y/lambda-generic-variadic3.C

+// PR c++/63628
+// { dg-do compile { target c++14 } }
+
+auto const pack = [](auto&&... t)
+{
+  return [&](auto&& f)->decltype(auto)
+  {
+    return f(static_cast<decltype(t)>(t)...);
+  };
+};
+
+int main(int argc, char** argv) {
+  pack(1)([](int){});
+  return 0;
+}