re PR c++/67364 ("accessing uninitialized member" error in constexpr context)

	PR c++/67364

	* constexpr.c (cxx_eval_component_reference): Further tweak.

From-SVN: r233982

gcc/cp/ChangeLog

 2016-03-04  Jason Merrill  <jason@redhat.com>
 
+	PR c++/67364
+	* constexpr.c (cxx_eval_component_reference): Further tweak.
+
 	* constexpr.c (struct constexpr_ctx): Add save_exprs field.
 	(cxx_eval_loop_expr): Discard SAVE_EXPR values before looping.
 	(cxx_eval_constant_expression) [SAVE_EXPR]: Add it to the set.

gcc/cp/constexpr.c

@@ -1990,13 +1990,16 @@ cxx_eval_component_reference (const constexpr_ctx *ctx, tree t,
       return t;
     }
 
-  if (CONSTRUCTOR_NO_IMPLICIT_ZERO (whole)
-      && !is_really_empty_class (TREE_TYPE (t)))
+  /* We only create a CONSTRUCTOR for a subobject when we modify it, so empty
+     classes never get represented; throw together a value now.  */
+  if (is_really_empty_class (TREE_TYPE (t)))
+    return build_constructor (TREE_TYPE (t), NULL);
+
+  if (CONSTRUCTOR_NO_IMPLICIT_ZERO (whole))
     {
       /* 'whole' is part of the aggregate initializer we're currently
 	 building; if there's no initializer for this member yet, that's an
-	 error.  But expand_aggr_init_1 doesn't bother to initialize really
-	 empty classes, so ignore them here, too.  */
+	 error.  */
       if (!ctx->quiet)
 	error ("accessing uninitialized member %qD", part);
       *non_constant_p = true;

gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C

+// { dg-do compile { target c++14 } }
+
+struct A
+{
+  constexpr A(int) { }
+};
+
+struct B: A {
+  constexpr B(int i): A(i) { }
+  constexpr B(const B& b): A(b) { }
+};
+
+struct C {
+  B b;
+  constexpr C(int i): b(i) { }
+  constexpr C(const C&c): b(c.b) {}
+};
+
+constexpr int f()
+{
+  C b1{42};
+  C b2{b1};
+  b2.b;
+  return 42;
+}
+
+constexpr int i = f();