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From: "jonathan.oconnor at protonmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/100612] std::jthread can't be initialized with a
 pointer to a member function taking a std::stop_token
Date: Sat, 15 May 2021 12:43:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100612

--- Comment #1 from Jonathan O'Connor <jonathan.oconnor at protonmail dot c=
om> ---
The std::jthread constructor does not support taking a pointer to a member
function that has, as a first argument, a std::stop_token.

In C++20, the new jthread class can accept a std::stop_token to aid in stop=
ping
a thread cooperatively. Unfortunately, the library code in gcc 10.2.0 and a=
ll
later versions (according to godbolt.org) do not allow a member function ta=
king
a stop_token to be used to initialize a jthread.

The problem is due to the std::jthread::_S_create() method which can't hand=
le
my desired case.

In a private email discussion with Nico Josuttis, his opinion was this shou=
ld
be allowed by the standard:
  "well, my rough understanding is that as INVOKE is called
  (see 20.9.2 Requirements [func.require] in the Standard),
  you can always pass a member function with the object to call the member
  function for as next argument.
  That should work for both thread and jthread."

The fix should be relatively simple, involving a further if constexpr check=
 for
the Callable being a member function. I'll try and make a patch, and append=
 it
here.

#include <thread>

struct ThreadObj {
    void withoutStopToken() {}
    void withStopToken(std::stop_token st) {}
};

int main() {
    ThreadObj obj;
    // The following line causes an error. The other example compile fine.
    std::jthread t1{&ThreadObj::withStopToken, &obj};
    std::jthread t2{&ThreadObj::withoutStopToken, &obj};
    return 0;
}=