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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?
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An incomplete type can be: 1 A structure type whose members you have not yet specified. 2 A union type whose members you have not yet specified. 3 An array type whose dimension you have not yet specified. More ...
For this class, a constructible type is a type that can be constructed using a particular set of arguments. is_constructible inherits from integral_constant as being either true_type or false_type, depending on whether T is constructible with the list of arguments Args. A complete type, or void (possible cv-qualified), or an array of unknown bound.
Args> struct is_constructible; For this class, a constructible type is a type that can be constructed using a particular set of arguments. is_constructible inherits from integral_constant as being either true_type or false_type, depending on whether T is constructible with the list of arguments Args.
To create an incomplete array type, declare an array type without specifying its repetition count. For example: To complete an incomplete array type, declare the same name later in the same scope with its repetition count specified, as in
The behavior is undefined.
[meta.unary.prop]
template <class T, class... Args> struct is_constructible;
Tand all types in the parameter packArgsshall 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.
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