[PATCH] Add support for C++2a stop_token

[Jonathan Wakely]

On 13/11/19 17:59 -0800, Thomas Rodgers wrote:
>+  // TODO verify the standard requires this
>+#if 0
>+  // register another callback
>+  bool cb3called{false};
>+  std::stop_callback scb3{stok, [&]
>+                                {
>+                                  cb3called = true;
>+                                }};
>+  VERIFY(ssrc.stop_possible());
>+  VERIFY(ssrc.stop_requested());
>+  VERIFY(stok.stop_possible());
>+  VERIFY(stok.stop_requested());
>+  VERIFY(!cb1called);
>+  VERIFY(cb2called);
>+  VERIFY(cb3called);
>+#endif

The working draft definitely requires this:

Effects: Initializes callback with std::forward<C>(cb). If st.stop_requested() is true, then
std::forward<Callback>(callback)() is evaluated in the current thread before the constructor
returns.