std::is_constructible on incomplete types

Question

I have the following code:

#include <iostream>  class A;  int main() {     std::cout << std::is_constructible<A>::value << std::endl; } 

When I use GCC 8.3, this code compiles. However, when I use Clang 8.0, I get a compilation error that incomplete types cannot be used in type traits.

Which one is correct? Am I allowed to use is_constructible on an incomplete type (with an expected value of false), or am I not allowed to?

Answer 1

The behavior is undefined.

[meta.unary.prop]

template <class T, class... Args> struct is_constructible; 

T and all types in the parameter pack Args shall be complete types, (possibly cv-qualified) void, or arrays of unknown bound.

That's a precondition of the meta-function. A contract that your code violates. libc++ is being generous by notifying you.

---

Mind you, that putting that precondition there and leaving it undefined otherwise is for a reason. A program where two points of instantiation of a template have different meanings is ill-formed NDR. The only sane course of action is demand complete types. And after all, that's when the trait is most useful anyway.