[/
 / Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:Scheduler Scheduler concept]

    template<class S>
      concept scheduler =
        copy_constructible<remove_cvref_t<S>> &&
        equality_comparable<remove_cvref_t<S>> &&
        requires(E&& e) {
          execution::schedule((E&&)e);
        };

None of a scheduler's copy constructor, destructor, equality comparison, or
`swap` operation shall exit via an exception.

None of these operations, nor a scheduler type's `schedule` function, or
associated query functions shall introduce data races as a result of concurrent
invocations of those functions from different threads.

For any two (possibly const) values `x1` and `x2` of some scheduler type `X`,
`x1 == x2` shall return `true` only if `asio::query(x1, p) == asio::query(x2,
p)` for every property `p` where both `asio::query(x1, p)` and `asio::query(x2,
p)` are well-formed and result in a non-void type that is `EqualityComparable`
(C++Std [equalitycomparable]). [inline_note The above requirements imply that
`x1 == x2` returns `true` if `x1` and `x2` can be interchanged with identical
effects. A scheduler may conceptually contain additional properties which are
not exposed by a named property type that can be observed via `asio::query`; in
this case, it is up to the concrete scheduler implementation to decide if these
properties affect equality. Returning `false` does not necessarily imply that
the effects are not identical.]

A scheduler type's destructor shall not block pending completion of any
receivers submitted to the sender objects returned from `schedule`.
[inline_note The ability to wait for completion of submitted function objects
may be provided by the execution context that produced the scheduler.]

In addition to the above requirements, type `S` models `scheduler` only if it
satisfies the requirements in the Table below.

In the Table below, 

* `s` denotes a (possibly const) scheduler object of type `S`,
* `N` denotes a type that models `sender`, and
* `n` denotes a sender object of type `N`

[table Scheduler requirements
  [[expression] [return type] [operation semantics]]
  [
    [`execution::schedule(s)`]
    [`N`]
    [Evaluates `execution::schedule(s)` on the calling thread to create `N`.]
  ]
]

`execution::start(o)`, where `o` is the result of a call to
`execution::connect(N, r)` for some receiver object `r`, is required to eagerly
submit `r` for execution on an execution agent that `s` creates for it. Let
`rc` be `r` or an object created by copy or move construction from `r`. The
semantic constraints on the `sender` `N` returned from a scheduler `s`'s
`schedule` function are as follows:

* If `rc`'s `set_error` function is called in response to a submission error,
  scheduling error, or other internal error, let `E` be an expression that
  refers to that error if `set_error(rc, E)` is well-formed; otherwise, let `E`
  be an `exception_ptr` that refers to that error. [Note: `E` could be the
  result of calling `current_exception` or `make_exception_ptr`.]
  The scheduler calls `set_error(rc, E)` on an unspecified weakly-parallel
  execution agent ([Note: An invocation of `set_error` on a receiver is
  required to be `noexcept`]), and

* If `rc`'s `set_error` function is called in response to an exception that
  propagates out of the invocation of `set_value` on `rc`, let `E` be
  `make_exception_ptr(receiver_invocation_error{})` invoked from within a catch
  clause that has caught the exception. The executor calls `set_error(rc, E)`
  on an unspecified weakly-parallel execution agent, and

* A call to `set_done(rc)` is made on an unspecified weakly-parallel execution
  agent ([Note: An invocation of a receiver's `set_done` function is required
  to be `noexcept`]).

[inline_note The senders returned from a scheduler's `schedule` function have
wide discretion when deciding which of the three receiver functions to call
upon submission.]

[endsect]