PR c++/79592 adjust testcase

https://gcc.gnu.org/ml/gcc-patches/2020-01/msg00127.html
	* g++.dg/ubsan/vptr-4.C: Add expected error.

From-SVN: r279902

gcc/testsuite/ChangeLog

+2020-01-06  Nathan Sidwell  <nathan@acm.org>
+
+	PR c++/79592
+	* g++.dg/ubsan/vptr-4.C: Add expected error.
+
 2020-01-06  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* gcc.target/aarch64/sve/acle/general/cpy_1.c: New test.

gcc/testsuite/g++.dg/ubsan/vptr-4.C

@@ -35,7 +35,7 @@ struct W : V<U> {
   int b;
   int g() { return 0; }
   virtual int v() { return 1; }
-  constexpr const W<U> *foo() { return (const W<U> *) reinterpret_cast<const V<U> *> (this); }
+  constexpr const W<U> *foo() { return (const W<U> *) reinterpret_cast<const V<U> *> (this); } // { dg-error "is not a constant expression" }
 };
 
 constexpr W<int> w;