diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index dc0d4a20814..fabcb1a5b4c 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2019-02-20  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/89336
+	* constexpr.c (cxx_eval_store_expression): Diagnose changing of active
+	union member for -std=c++17 and earlier.
+
 2019-02-19  Jason Merrill  <jason@redhat.com>
 
 	PR c++/87513 - 'sorry' mangling PMF template-id.
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index d413c6b9b27..a9aa7d50041 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3813,8 +3813,20 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 
 	  if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
 	      && CONSTRUCTOR_ELT (*valp, 0)->index != index)
-	    /* Changing active member.  */
-	    vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
+	    {
+	      if (cxx_dialect < cxx2a)
+		{
+		  if (!ctx->quiet)
+		    error_at (cp_expr_loc_or_loc (t, input_location),
+			      "change of the active member of a union "
+			      "from %qD to %qD",
+			      CONSTRUCTOR_ELT (*valp, 0)->index,
+			      index);
+		  *non_constant_p = true;
+		}
+	      /* Changing active member.  */
+	      vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
+	    }
 
 	  for (idx = 0;
 	       vec_safe_iterate (CONSTRUCTOR_ELTS (*valp), idx, &cep);
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 9d344ba7c2d..6be764f6ddb 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2019-02-20  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/89336
+	* g++.dg/cpp1y/constexpr-89336-3.C: New test.
+
 2019-02-20  David Malcolm  <dmalcolm@redhat.com>
 
 	PR c/89410
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
new file mode 100644
index 00000000000..9d370ddefcf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C
@@ -0,0 +1,46 @@
+// PR c++/89336
+// { dg-do compile { target c++14 } }
+
+constexpr int
+foo ()
+{
+  union U { int a; long b; };
+  union V { union U u; short v; };
+  V w {};
+  w.u.a = w.v = w.u.b = 5L;		// { dg-error "change of the active member of a union from" "" { target c++17_down } }
+  return w.u.a;
+}
+
+static_assert (foo () == 5, "");	// { dg-error "non-constant condition for static assertion" "" { target c++17_down } }
+					// { dg-message "expansion of" "" { target c++17_down } .-1 }
+
+constexpr int
+bar ()
+{
+  union U { int a[5]; long b; };
+  union V { union U u; short v; };
+  V w {};
+  w.v = 5;
+  w.u.a[3] = w.u.a[1] = w.v;		// { dg-error "change of the active member of a union from" "" { target c++17_down } }
+  return w.u.a[1] + w.u.a[3];
+}
+
+static_assert (bar () == 10, "");	// { dg-error "non-constant condition for static assertion" "" { target c++17_down } }
+					// { dg-message "expansion of" "" { target c++17_down } .-1 }
+
+struct Z { int x, y; };
+
+constexpr Z
+baz ()
+{
+  union W { Z a; long long w; };
+  W w {};
+  w.a = { 5, 0 };
+  w.a = { (int) (w.w = 17LL + w.a.x), 2 };	// { dg-error "change of the active member of a union from" "" { target c++17_down } }
+  return w.a;
+}
+
+static_assert (baz ().x == 22, "");	// { dg-error "non-constant condition for static assertion" "" { target c++17_down } }
+					// { dg-message "expansion of" "" { target c++17_down } .-1 }
+static_assert (baz ().y == 2, "");	// { dg-error "non-constant condition for static assertion" "" { target c++17_down } }
+					// { dg-message "expansion of" "" { target c++17_down } .-1 }