c++: no_unique_address and constexpr [PR112439]

Here, because we don't build a CONSTRUCTOR for an empty base, we were
wrongly marking the Foo CONSTRUCTOR as complete after initializing the Empty
member.  Fixed by checking empty_base here as well.

	PR c++/112439

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_store_expression): Check empty_base
	before marking a CONSTRUCTOR readonly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/no_unique_address15.C: New test.

gcc/cp/constexpr.cc

@@ -6694,6 +6694,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
      object.  Make a note of this fact by marking the CONSTRUCTOR
      TREE_READONLY.  */
   if (TREE_CODE (t) == INIT_EXPR
+      && !empty_base
       && TREE_CODE (*valp) == CONSTRUCTOR
       && TYPE_READONLY (type))
     {

gcc/testsuite/g++.dg/cpp2a/no_unique_address15.C

@@ -0,0 +1,19 @@
+// PR c++/112439
+// { dg-do compile { target c++14 } }
+
+struct Empty {};
+
+class Foo {
+public:
+    constexpr Foo(int x, Empty y, int z) : a(x), b(y)
+    {
+        c = z;
+    }
+
+private:
+    int a{};
+    [[no_unique_address]] Empty b{};
+    [[no_unique_address]] int c{};
+};
+
+constexpr Foo r{1, {}, 3};