diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 68c5d9d1cd9..be51428cc5b 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,9 @@
 2017-02-10  Jason Merrill  <jason@redhat.com>
 
+	PR c++/78897 - constexpr union
+	* constexpr.c (cxx_eval_store_expression): A store to a union member
+	erases a previous store to another member.
+
 	PR c++/71285 - member of fold-expression
 	* semantics.c (finish_unary_fold_expr)
 	(finish_binary_fold_expr): Use null type for fold-expressions.
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index bb45f1e8059..bfdde9e911d 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -3466,6 +3466,11 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
 	  tree fields = TYPE_FIELDS (DECL_CONTEXT (index));
 	  unsigned HOST_WIDE_INT idx;
 
+	  if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
+	      && CONSTRUCTOR_ELT (*valp, 0)->index != index)
+	    /* Changing active member.  */
+	    vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
+
 	  for (idx = 0;
 	       vec_safe_iterate (CONSTRUCTOR_ELTS (*valp), idx, &cep);
 	       idx++, fields = DECL_CHAIN (fields))
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-union1.C
new file mode 100644
index 00000000000..8aed6d9bf55
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union1.C
@@ -0,0 +1,11 @@
+// PR c++/78897
+// { dg-do compile { target c++14 } }
+
+struct Optional {
+  constexpr Optional() : _dummy{} { _value = 1; }
+  union {
+    int _dummy;
+    int _value;
+  };
+};
+Optional opt{};