code: def test_warns(self, xp, c_dtype, f_dtype):
      with pytest.warns(numpy.ComplexWarning):
          out = xp.ones((8,), dtype=c_dtype).sum(dtype=f_dtype)
      return out
  
imports: import unittest
  import numpy
  import pytest
  import cupy
  import cupy._core._accelerator
  from cupy import _core
  from cupy import testing
label_desc: Use image node_id 3 for calling the TestSimpleReductionFunctionComplexWarning obj's underlying member method code with example usage: obj.test_warns(xp, c_dtype, f_dtype) and returns: out