std::jthread::operator=(std::jthread&&) calls std::terminate if *this has an associated running thread.

[Paul Scharnofske <asynts@gmail.com>]

Disclaimer 1: That C++ Standard stuff seems really complicated, I only 
really looked at
https://cppreference.com and then verified it with 
https://github.com/cplusplus/draft . I hope
I got this right.

Disclaimer 2: I've never send anything to a mailing list, I hope this works.

The following snippet does the wrong thing:

     #include <thread>

     int main() {
         std::jthread thread { [] {} };

         // Calls std::terminate in std::thread::operator=(std::thread&&).
         // Should instead join the previous thread before doing the 
assignment.
         thread = std::jthread{ [] {} };
     }

This looks like an oversight to me, from the standard:

     32.4.4.2 Constructors, move, and assignment [thread.jthread.cons]

     jthread& operator=(jthread&& x) noexcept;

         Effects: If joinable() is true, calls request_stop() and then 
join().
         Assigns the state of x to *this and sets x to a default constructed
         state. [...]

     [...]

     32.4.3.5 Assignment [thread.thread.assign]

     thread& operator=(thread&& x) noexcept;

         Effects: If joinable(), invokes terminate (14.6.2). Otherwise, 
assigns
         the state of x to *this and sets x to a default constructed state.
         [...]

I suggest the following fix: (mirror: 
https://static.asynts.com/2020/11/06/jthread.patch)

     diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
     index 89f9f6c8c38..02e4c3cc8a6 100644
     --- a/libstdc++-v3/ChangeLog
     +++ b/libstdc++-v3/ChangeLog
     @@ -1,3 +1,8 @@
     +2020-11-06  Paul Scharnofske  <asynts@gmail.com>
     +
     +       * include/std/thread (operator=(std::jthread&&): Join 
current thread if it
     +       is running before moving it.
     +
      2020-11-05  Marek Polacek  <polacek@redhat.com>

             PR c++/25814
     diff --git a/libstdc++-v3/include/std/thread 
b/libstdc++-v3/include/std/thread
     index 887ee579962..773befa75ad 100644
     --- a/libstdc++-v3/include/std/thread
     +++ b/libstdc++-v3/include/std/thread
     @@ -456,7 +456,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          operator=(const jthread&) = delete;

          jthread&
     -    operator=(jthread&&) noexcept = default;
     +    operator=(jthread&& __other) noexcept
     +    {
     +      if (joinable())
     +        {
     +          request_stop();
     +
     +          // The C++ Standard (working draft) says that this 
method must be
     +          // noexcept, but also dictates that join be called. It 
doesn't say
     +          // how to do this, this is probably the way to go?
     +          try
     +            {
     +              join();
     +            }
     +          catch (...)
     +            {
     +              std::terminate();
     +            }
     +        }
     +
     +      swap(__other);
     +
     +      return *this;
     +    }

          void
          swap(jthread& __other) noexcept

Like already mentioned in the comment, there is a bit of an inconsistency in
the Standard since the move assignment operator is marked as noexcept 
and the join
function can throw an exception...

I don't know if there is something in the standard that dictates what 
should happen in such
a case, but I really don't know where to look for it.