PR c++/88690 - C++17 ICE with empty base in aggregate.

Base fields for empty bases appear in initialization order, which may not be
the same as layout order.  If they also show up in a CONSTRUCTOR in that
order, output_constructor_regular_field aborts because it understandably
doesn't want to go backwards.  I also considered making o_c_r_f more
tolerant of the case where the out-of-order field has fieldsize 0, and so no
actual data needs to be emitted, but we might as well avoid adding an
element to the CONSTRUCTOR in the first place.

	* typeck2.c (process_init_constructor_record): Skip trivial
	initialization of an empty base.

From-SVN: r269073

gcc/cp/ChangeLog

+2019-02-20  Jason Merrill  <jason@redhat.com>
+
+	PR c++/88690 - C++17 ICE with empty base in aggregate.
+	* typeck2.c (process_init_constructor_record): Skip trivial
+	initialization of an empty base.
+
 2019-02-21  Richard Biener  <rguenther@suse.de>
 
 	PR middle-end/89392

gcc/cp/typeck2.c

@@ -1627,6 +1627,13 @@ process_init_constructor_record (tree type, tree init, int nested, int flags,
 	    }
 	}
 
+      if (DECL_SIZE (field) && integer_zerop (DECL_SIZE (field))
+	  && !TREE_SIDE_EFFECTS (next))
+	/* Don't add trivial initialization of an empty base/field to the
+	   constructor, as they might not be ordered the way the back-end
+	   expects.  */
+	continue;
+
       /* If this is a bitfield, now convert to the lowered type.  */
       if (type != TREE_TYPE (field))
 	next = cp_convert_and_check (TREE_TYPE (field), next, complain);

gcc/testsuite/g++.dg/cpp1z/aggr-base7.C

+// PR c++/88690
+// { dg-do compile { target c++11 } }
+
+struct A { int a = 1; };
+struct B { int b = 0; };
+struct C { C() = default; C (const C&) = delete; };
+struct D : public B, public C {};
+struct E : A { D f; } g{};