fixes

nifty5/operators/operator.py

@@ -219,10 +219,16 @@ class _ConstantOperator(Operator):
     def apply(self, x):
         from ..linearization import Linearization
         from .simple_linear_operators import NullOperator
+        from ..domain_tuple import DomainTuple
         self._check_input(x)
         if not isinstance(x, Linearization):
             return self._output
-        return x.new(self._output, NullOperator(self._domain, self._target))
+        if x.want_metric and self._target is DomainTuple.scalar_domain():
+            met = NullOperator(self._domain, self._domain)
+        else:
+            met = None
+        return x.new(self._output, NullOperator(self._domain, self._target),
+                     met)
 
     def __repr__(self):
         return 'ConstantOperator <- {}'.format(self.domain.keys())

test/test_operators/test_simplification.py

@@ -48,3 +48,8 @@ def test_simplification():
     assert_equal(isinstance(op2._op1, _ConstantOperator), True)
     assert_allclose(op(f1)["a"].local_data, op2(f1)["a"].local_data)
     assert_allclose(op(f1)["b"].local_data, op2(f1)["b"].local_data)
+    lin = ift.Linearization.make_var(ift.MultiField.full(op2.domain, 2.), True)
+    assert_allclose(op(lin).val["a"].local_data,
+                    op2(lin).val["a"].local_data)
+    assert_allclose(op(lin).val["b"].local_data,
+                    op2(lin).val["b"].local_data)