async_range.h

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#pragma once
 
#include "di/execution/coroutine/env_promise.h"
#include "di/execution/meta/await_result.h"
#include "di/execution/types/empty_env.h"
#include "di/function/invoke.h"
#include "di/meta/vocab.h"
 
namespace di::execution {
namespace next_ns {
    struct Function {
        template<typename AsyncRange>
        requires(concepts::TagInvocable<Function, AsyncRange&>)
        auto operator()(AsyncRange& async_range) const {
            static_assert(
                concepts::IsAwaitable<meta::TagInvokeResult<Function, AsyncRange&>, EnvPromise<types::EmptyEnv>>,
                "The return value of execution::next() must be awaitable with an awaitable.");
            return function::tag_invoke(*this, async_range);
        }
    };
}

constexpr inline auto next = next_ns::Function {};
}
 
namespace di::concepts {
template<typename T>
concept AsyncRange = concepts::Optional<
    meta::AwaitResult<meta::InvokeResult<decltype(execution::next), T&>, execution::EnvPromise<types::EmptyEnv>>>;
 
template<typename T>
concept AwaitableAsyncRange = AsyncRange<meta::AwaitResult<T, execution::EnvPromise<types::EmptyEnv>>>;
}