std/d/ast.d DeclarationOrStatement Assert fails when using --xml

mixin (visitIfNotNull!(declaration, statement));

fails on phobos:

std/algorithm.d std/array.d std/bitmanip.d std/container.d std/conv.d std/csv.d std/datetime.d std/file.d std/format.d std/math.d std/numeric.d std/outbuffer.d std/parallelism.d std/path.d std/process.d std/random.d std/range.d std/regex.d std/stdio.d std/stream.d std/string.d std/uni.d std/uri.d std/utf.d std/xml.d
