Extended Description

C++20 clarifies the expectation for detecting traits that cannot determine a result due to an incomplete class. However, is_base_of is expected to recognize that unions can never have a base class, nor ever be a base class, and so give the correct result - per the current working draft.

The following code is expected to compile without triggering a static assert. For the current clang/libc++, it fails to compile the middle two heterogeneous assertions:

#include <type_traits>

struct Incomplete;
union Uncomplete;

int main() {
static_assert( std::is_base_of_v<Incomplete, Incomplete>);
static_assert(!std::is_base_of_v<Incomplete, Uncomplete>); // fails to compile
static_assert(!std::is_base_of_v<Uncomplete, Incomplete>); // fails to compile
static_assert(!std::is_base_of_v<Uncomplete, Uncomplete>);
}