if (typeof then === 'function') {
    let d;

    FunctionPrototypeCall(
      then,
      body,
      (val) => {
        if (val != null) {
          d.push(val);
        }
        d.push(null);
      },
      (err) => {
        destroyer(d, err);
      }
    );

    return d = new Duplexify({
      objectMode: true,
      writable: false,
      read() {}
    });
  }

  throw new ERR_INVALID_ARG_TYPE(
    name,
    ['Blob', 'ReadableStream', 'WritableStream', 'Stream', 'Iterable',
     'AsyncIterable', 'Function', '{ readable, writable } pair', 'Promise'],
    body);
};

function fromAsyncGen(fn) {
  let { promise, resolve } = createDeferredPromise();
  const ac = new AbortController();
  const signal = ac.signal;
  const value = fn(async function*() {
    while (true) {
      const _promise = promise;
      promise = null;
      const { chunk, done, cb } = await _promise;
      process.nextTick(cb);
      if (done) return;
      if (signal.aborted) throw new AbortError();
      ({ promise, resolve } = createDeferredPromise());
      yield chunk;
    }
  }(), { signal });

  return {
    value,
    write(chunk, encoding, cb) {
      const _resolve = resolve;
      resolve = null;
      _resolve({ chunk, done: false, cb });
    },
    final(cb) {
      const _resolve = resolve;
      resolve = null;
      _resolve({ done: true, cb });
    },
    destroy(err, cb) {
      ac.abort();
      cb(err);
    }
  };
}

function _duplexify(pair) {
  const r = pair.readable && typeof pair.readable.read !== 'function' ?
    Readable.wrap(pair.readable) : pair.readable;
  const w = pair.writable;

  let readable = !!isReadable(r);
  let writable = !!isWritable(w);

  let ondrain;
  let onfinish;
  let onreadable;
  let onclose;
  let d;

  function onfinished(err) {
    const cb = onclose;
    onclose = null;

    if (cb) {
      cb(err);
    } else if (err) {
      d.destroy(err);
    } else if (!readable && !writable) {
      d.destroy();
    }
  }

  // TODO(ronag): Avoid double buffering.
  // Implement Writable/Readable/Duplex traits.
  // See, https://github.com/nodejs/node/pull/33515.
  d = new Duplexify({
    // TODO (ronag): highWaterMark?
    readableObjectMode: !!r?.readableObjectMode,
    writableObjectMode: !!w?.writableObjectMode,
    readable,
    writable,
  });

  if (writable) {
    eos(w, (err) => {
      writable = false;
      if (err) {
        destroyer(r, err);
      }
      onfinished(err);
    });

    d._write = function(chunk, encoding, callback) {
      if (w.write(chunk, encoding)) {
        callback();
      } else {
        ondrain = callback;
      }
    };

    d._final = function(callback) {
      w.end();
      onfinish = callback;
    };

    w.on('drain', function() {
      if (ondrain) {
        const cb = ondrain;
        ondrain = null;
        cb();
      }
    });

    w.on('finish', function() {
      if (onfinish) {
        const cb = onfinish;
        onfinish = null;
        cb();
      }
    });
  }

  if (readable) {
    eos(r, (err) => {
      readable = false;
      if (err) {
        destroyer(r, err);
      }
      onfinished(err);
    });

    r.on('readable', function() {
      if (onreadable) {
        const cb = onreadable;
        onreadable = null;
        cb();
      }
    });

    r.on('end', function() {
      d.push(null);
    });

    d._read = function() {
      while (true) {
        const buf = r.read();

        if (buf === null) {
          onreadable = d._read;
          return;
        }

        if (!d.push(buf)) {
          return;
        }
      }
    };
  }

  d._destroy = function(err, callback) {
    if (!err && onclose !== null) {
      err = new AbortError();
    }

    onreadable = null;
    ondrain = null;
    onfinish = null;

    if (onclose === null) {
      callback(err);
    } else {
      onclose = callback;
      destroyer(w, err);
      destroyer(r, err);
    }
  };

  return d;
}