+// PR c++/89481
+// { dg-do compile { target c++14 } }
+
+constexpr int
+foo ()
+{
+ union U { long long a; int b[2]; } u { 5LL };
+ u.b[1] = 4; // { dg-error "change of the active member of a union from" "" { target c++17_down } }
+ return u.b[0];
+}
+
+constexpr int
+bar ()
+{
+ union U { long long a; int b[2]; } u { 5LL };
+ u.b[1] = 4; // { dg-error "change of the active member of a union from" "" { target c++17_down } }
+ return u.b[1];
+}
+
+static_assert (foo () == 0, ""); // { dg-error "non-constant condition for static assertion" }
+ // { dg-message "in 'constexpr' expansion of" "" { target *-*-* } .-1 }
+ // { dg-error "accessing uninitialized array element" "" { target c++2a } .-2 }
+static_assert (bar () == 4, ""); // { dg-error "non-constant condition for static assertion" "" { target c++17_down } }
+ // { dg-message "in 'constexpr' expansion of" "" { target c++17_down } .-1 }