option nogil=True not required for ufuncs and gufuncs

numba/np/ufunc/ufuncbuilder.py

@@ -2,6 +2,7 @@
 
 import inspect
 from contextlib import contextmanager
+import warnings
 
 from numba.core.decorators import jit
 from numba.core.descriptors import TargetDescriptor
@@ -82,6 +83,10 @@ def compile(self, sig, locals={}, **targetoptions):
         topt.update(targetoptions)
 
         flags = compiler.Flags()
+        if 'nogil' in topt:
+            warnings.warn("nogil option is not required, ufuncs and gufuncs "
+                          "always releases GIL for CPU target")
+            topt.pop('nogil')
         self.targetdescr.options.parse_as_flags(flags, topt)
 
         flags.set("no_cpython_wrapper")

numba/tests/test_warnings.py

@@ -8,6 +8,7 @@
 from numba import jit
 from numba.core.errors import NumbaWarning, deprecated, NumbaDeprecationWarning
 from numba.core import errors
+from numba import vectorize
 
 
 class TestBuiltins(unittest.TestCase):
@@ -204,6 +205,23 @@ def test_disable_performance_warnings(self):
         out, err = popen.communicate()
         self.assertEqual(popen.returncode, not_found_ret_code)
 
+    def test_warning_vectorize_with_nogil(self):
+        @vectorize(nogil=True)
+        def f(a, b):
+            return np.sqrt(a) + np.cos(b) * np.sin(b) - a
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('always')
+
+            nelem = 10 ** 2
+            x = np.arange(nelem)
+            f(x, x)
+
+            self.assertEqual(w[0].category, UserWarning)
+            self.assertIn('nogil option is not required', str(w[0].message))
+            self.assertIn('ufuncs and gufuncs always releases GIL for CPU '
+                          'target', str(w[0].message))
+
 
 if __name__ == '__main__':
     unittest.main()