in fatal/container/legacy_variant.h [879:893]
void operator ()(
indexed_type_tag<U, Index>, UTag &&tag, UVariant &&variant, bool &result,
UVisitor &&visitor, UArgs &&...args
) {
assert(tag == variant.tag()), (void) tag;
assert(Index == variant.tag());
using variant_type = typename std::decay<UVariant>::type;
static_assert(sizeof(variant_type) >= 0, "");
result = visit<UCondition<U>::value>(
variant.variant_type::template get<U>(),
std::forward<UVisitor>(visitor),
std::forward<UArgs>(args)...
);
}