test masked assign views (#4599)

* possible masked

* not contiguous mask

test/test_assign.py

@@ -345,6 +345,22 @@ def test_permuted_reduceop_multioutput_dual_use_possible(self):
     np.testing.assert_equal(b.numpy(), a.numpy().sum(1) + np.arange(32 * 32).reshape(32, 32))
     np.testing.assert_equal(c.numpy(), a.numpy().sum(1) + np.arange(32 * 32).reshape(32, 32).transpose(1, 0))
 
+  def test_permuted_assignment_masked_view_possible(self):
+    a = Tensor.ones(4, 4).contiguous().realize()
+    b = a.shrink((None, (0, 2))).pad((None, (0, 2)), 2)
+    a.assign(a + b)
+    kc = GlobalCounters.kernel_count
+    a.realize()
+    assert GlobalCounters.kernel_count - kc == 1
+    np.testing.assert_equal(a.numpy(), np.ones((4, 4))+np.pad(np.ones((4, 4))[:, 0:2], ((0, 0), (0, 2)), constant_values=2))
+
+  def test_permuted_assignment_masked_view_not_contiguous(self):
+    a = Tensor.ones(4, 4).contiguous().realize()
+    with self.assertRaisesRegex(RuntimeError, "contiguous"):
+      b = a.shrink((None, (0, 2))).pad((None, (0, 2)), 2).permute(1, 0)
+      a.assign(a + b)
+      a.realize()
+
   # TODO: is there a way to sneak in a permute such that it returns the wrong answer?
 
   @unittest.skip("don't use output buffer, and mismatch dtype no longer supported")