std::is_assignable and std::pair

Question

As expected, the following code does not compile.

#include <type_traits>
#include <utility>
int main()
{
    using T = std::pair<const int, int>;
    const auto ok = std::is_assignable<T, T>::value; // true
    T x;
    T y;
    x = y; // compiler error
}

But the value of ok is true with the following three compilers.

g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
MSVC++ 2017 15.2 26430.6

Why is this?

Can you try to compile it with a system that gives you the actual error messages, and show them to us? — Some programmer dude, Jun 14 '17 at 08:04
How did you deduce it's true exactly? A static assertion on OK fails. http://coliru.stacked-crooked.com/a/aba0debbfc609c11 — StoryTeller - Unslander Monica, Jun 14 '17 at 08:06
`std::cout << std::boolalpha << ok << '\n';` outputs `false` with Clang 5.0. — Henri Menke, Jun 14 '17 at 08:06
Also, if the code you show doesn't compile, how do you know that `ok` is `true`? Because [I can't replicate it](http://ideone.com/tlPkfl). — Some programmer dude, Jun 14 '17 at 08:06
Just added a link to my question showing ok is true. It's the same in MSVC++ 2015. — Tobias Hermann, Jun 14 '17 at 08:15
https://godbolt.org/g/cMfcBP As Holt said, it is true for gcc <6.3 so it looks like a bug — bolov, Jun 14 '17 at 08:17

T.C. · Accepted Answer · 2018-02-27T02:26:37.050

is_assignable asks the question "is there an assignment operator signature that accepts these arguments", not "will that assignment operator actually compile" (in standardese, it only considers the immediate context of the assignment expression):
```
template<class T>
struct foo {
    T t {};
    foo& operator=(const foo& r) { t = r.t; };
};
static_assert(std::is_copy_assignable<foo<const int>>::value, ""); // OK

void bar() {
    foo<const int> f1, f2;
    f1 = f2; // explodes
}
```
pair's assignment operators can't be defaulted, because it needs to do something special when the pair contains a reference. That means that additional precautions need to be taken to ensure that is_assignable doesn't lie (e.g., making sure that the copy assignment operator is deleted if a member type is not copy assignable). The standard has not mandated such precautions until very recently.
is_assignable<T, T> asks whether a T rvalue can be assigned to a T rvalue. This is an odd question to ask.

std::is_assignable and std::pair

1 Answers1

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