c++: Fix [[no_unique_address]] and default mem-init [PR90432]

output_constructor doesn't like two consecutive entries with fields at the
same position; let's avoid adding the one for the empty field.

gcc/cp/ChangeLog
2020-03-04  Jason Merrill  <jason@redhat.com>

	PR c++/90432
	* init.c (perform_member_init): Don't do aggregate initialization of
	empty field.
	* constexpr.c (cx_check_missing_mem_inits): Don't enforce
	initialization of empty field.

gcc/cp/ChangeLog

@@ -1,3 +1,11 @@
+2020-03-04  Jason Merrill  <jason@redhat.com>
+
+	PR c++/90432
+	* init.c (perform_member_init): Don't do aggregate initialization of
+	empty field.
+	* constexpr.c (cx_check_missing_mem_inits): Don't enforce
+	initialization of empty field.
+
 2020-03-04  Martin Liska  <mliska@suse.cz>
 
 	* method.c: Wrap array in ctor with braces in order

gcc/cp/constexpr.c

@@ -831,6 +831,9 @@ cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
 	    /* A flexible array can't be intialized here, so don't complain
 	       that it isn't.  */
 	    continue;
+	  if (DECL_SIZE (field) && integer_zerop (DECL_SIZE (field)))
+	    /* An empty field doesn't need an initializer.  */
+	    continue;
 	  ftype = strip_array_types (ftype);
 	  if (type_has_constexpr_default_constructor (ftype))
 	    {

gcc/cp/init.c

@@ -865,6 +865,11 @@ perform_member_init (tree member, tree init)
 	}
       if (init == error_mark_node)
 	return;
+      if (DECL_SIZE (member) && integer_zerop (DECL_SIZE (member))
+	  && !TREE_SIDE_EFFECTS (init))
+	/* Don't add trivial initialization of an empty base/field, as they
+	   might not be ordered the way the back-end expects.  */
+	return;
       /* A FIELD_DECL doesn't really have a suitable lifetime, but
 	 make_temporary_var_for_ref_to_temp will treat it as automatic and
 	 set_up_extended_ref_temp wants to use the decl in a warning.  */

gcc/testsuite/g++.dg/cpp2a/no_unique_address3.C

@@ -0,0 +1,16 @@
+// PR c++/90432
+// { dg-do compile { target c++11 } }
+
+struct empty {};
+
+struct has_empty {
+  [[no_unique_address]] empty brace_or_equal_initialized{};
+};
+
+struct has_value {
+  int non_zero = 1;
+};
+
+struct pair : has_empty, has_value {};
+
+pair a;