std::is_base_of should give correct result for incomplete unions

[AlisdairM]

Bugzilla Link	41843
Resolution	FIXED
Resolved on	Jun 17, 2019 08:45
Version	8.0
OS	All
Attachments	Source for failing test
CC	@mclow,@zygoloid

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>);
}

[AlisdairM]

assigned to @mclow

[mclow]

Do you remember the paper/issue that clarified this?
I can find it, but you probably have it on the tip of your fingers.

[AlisdairM]

I think we were both thinking of http://wg21.link/p1285 but it turns out not to be relevant, as that was about pointers and references to incomplete types. I believe the standard wording has required properly supporting incomplete union types in this specific trait for some time.

[mclow]

Whenever possible, libc++ calls the compiler intrinsic __is_base_of to have the compiler do the work.

[mclow]

Running the test w/o using the compiler intrinsic produces the expected results.

[mclow]

Both gcc and MSVC compile the code that Alisdair posted w/o error.

[mclow]

Patch up for review: https://reviews.llvm.org/D61858

[mclow]

resolved by revision 360614.