SFINAE with std::enable_if and std::is_default_constructible for incomplete type in libc++

Question

I just observed a strange issue with libc++ when using SFINAE to detect if a templated type is default constructible.

The following is a minimal example I was able to come up with:

#include <iostream>
#include <type_traits>

template <typename T>
struct Dummy;

template <>
struct Dummy<int>{};

template <typename T, typename = void>
struct has_dummy : std::false_type {};

template <typename T>
struct has_dummy<C, std::enable_if_t<std::is_default_constructible<Dummy<T>>::value>> : std::true_type{};

int main() {
    std::cout << std::boolalpha << has_dummy<int>{}() << '\n';
    std::cout << std::boolalpha << has_dummy<double>{}() << '\n';
}

It compiles and outputs the expected lines true and false when compiled with g++ or clang++ when using libstdc++. However when I try to compile it with libc++ (i.e. clang++ -stdlib=libc++ -std=c++1z test.cpp) I get the following error:

/usr/bin/../include/c++/v1/type_traits:2857:38: error: implicit instantiation of undefined template 'Dummy' : public integral_constant

/usr/bin/../include/c++/v1/type_traits:3166:14: note: in instantiation of template class 'std::__1::is_constructible>' requested here : public is_constructible<_Tp>

test.cpp:14:43: note: in instantiation of template class 'std::__1::is_default_constructible >' requested here struct has_dummy<T, std::enable_if_t<std::is_default_constructible<Dummy<T>>::value>> : std::true_type{};

test.cpp:18:35: note: during template argument deduction for class template partial specialization 'has_dummy<type-parameter-0-0, typename enable_if<std::is_default_constructible<Dummy<T> >::value, void>::type>' [with T = double]

   std::cout << std::boolalpha << has_dummy<double>{}() << '\n';

test.cpp:5:8: note: template is declared here struct Dummy;

Is this a bug in libc++'s implementation of std::enable_if or std::is_default_constructible or is what I'm doing somehow invoking undefined/implementation specific behaviour?

Best Corristo

Answer 1

Preconditions for is_default_constructible state quite clearly:

N4140 § 20.10.4.3 [meta.unary.prop] / is_default_constructible row

T shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound.

And according to the following, your program exhibits undefined behaviour:

N4140 § 17.6.4.8 [res.on.functions] / 2

the effects are undefined in the following cases:

• [...]
• if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for that component.