C++ PATCH to tweak variadic126.C test (PR c++/88866)

[Marek Polacek]

This test was failing in c++2a because the error messages are different
because we're now trying to parse braced-list in template-argument-list.

Tested on x86_64-linux, ok for trunk?

2019-01-15  Marek Polacek  <pola...@redhat.com>

        PR c++/88866
        * g++.dg/cpp0x/variadic126.C: Tweak dg-error.

diff --git gcc/testsuite/g++.dg/cpp0x/variadic126.C 
gcc/testsuite/g++.dg/cpp0x/variadic126.C
index 513c7e54a07..a219309f058 100644
--- gcc/testsuite/g++.dg/cpp0x/variadic126.C
+++ gcc/testsuite/g++.dg/cpp0x/variadic126.C
@@ -3,6 +3,6 @@
 
 template < typename ... > struct A;
 
-struct B : A <                 // { dg-error "invalid" }
+struct B : A <                 // { dg-error "invalid" "" { target c++17_down 
} }
 {
-};
+}; // { dg-error "mismatch|expected" "" { target c++2a } }