为何Boost callable_traits的is_invocable_v与is_invocable::value返回结果不同？

This inconsistency you're seeing is a known bug in Boost.CallableTraits 1.72.

In this version, the variable template boost::callable_traits::is_invocable_v was incorrectly implemented — it didn't properly reference the value member of the is_invocable trait. Instead, it likely pointed to an unrelated type or had a wrong definition, leading to the false (0) result while the underlying is_invocable<...>::value works correctly (matching std::is_invocable's behavior).

Solutions to fix this: #

• Define your own variable template as a workaround:

  template<typename Func, typename... Args>
  constexpr bool boost_is_invocable_v = boost::callable_traits::is_invocable<Func, Args...>::value;
  

  You can use this custom boost_is_invocable_v in place of the broken library-provided one.

• Upgrade your Boost version to 1.73 or later. This bug was fixed in subsequent releases of Boost.CallableTraits, so the is_invocable_v variable template will behave consistently with both is_invocable::value and std::is_invocable_v.

To confirm, if you test your code with Boost 1.73+, you'll see all four outputs return 1 as expected.

内容的提问来源于stack exchange，提问作者maxbachmann