[cfe-dev] What does __is_constructible actually check?

[Marshall Clow]

Consider this code:

#include <iostream>
#include <cassert>

struct Abstract {
	Abstract () {}
	~Abstract () {}
	virtual bool operator() () const = 0;
	};

struct Private {
private:
	Private () {}
	~Private () {}
	};

int
main()
{
	std::cout << __is_constructible(int) << std::endl;
	std::cout << __is_constructible(Abstract) << std::endl;
	std::cout << __is_constructible(Private) << std::endl;
}


What would you expect this to print?
I would expect:
	1
	0
	0

but instead, it prints.
	Has __is_constructible
	1
	1
	0

In other words, __is_constructible(Abstract) returns true.
But I can’t actually _construct_ an instance of Abstract:

junk2.cpp:27:11: error: variable type 'Abstract' is an abstract class
        Abstract a;
                 ^
junk2.cpp:7:15: note: unimplemented pure virtual method 'operator()' in
      'Abstract'
        virtual bool operator() () const = 0;
                     ^
1 error generated.

Is this behavior deliberate? Or just a bug?

— Marshall