Why is vector considered nothrow_move_constructible?

Question

The specification for vector's move constructor is (copied out of the standard):

vector(vector&&);

Notice the lack of noexcept. But both gcc 4.8 and Clang 3.2 report that std::is_nothrow_move_constructible<std::vector<int>>::value returns true (i.e, 1):

#include<vector>
#include<iostream>

int main()
{
  std::cout << std::is_nothrow_move_constructible<std::vector<int>>::value << '\n';
}

What is the cause of this apparent discrepancy?

Answer 1

The standard allows an implementation to strengthen the exception specification of a method as per

17.6.5.12 Restrictions on exception handling [res.on.exception.handling]

⁴ Destructor operations defined in the C++ standard library shall not throw exceptions. Every destructor in the C++ standard library shall behave as if it had a non-throwing exception specification. Any other functions defined in the C++ standard library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified.¹⁹¹ An implementation may strengthen this implicit exception-specification by adding an explicit one.¹⁹²