Modified scalar.Identity to provide casting functionality

theano/scalar/basic.py

@@ -805,12 +805,12 @@ second = Second(transfer_type(1), name = 'second')
 
 class Identity(UnaryScalarOp):
     def impl(self, x):
-        return x
+        return getattr(numpy, self.output_types_preference.spec[0].dtype)(x)
     def c_code(self, node, name, (x, ), (z, ), sub):
         return "%(z)s = %(x)s;" % locals()
     def grad(self, (x, ), (gz, )):
         if x.type in grad_types:
-          return gz,
+          return [cast(gz, x.type.dtype)]
         else:
           return None,
 
@@ -841,6 +841,8 @@ def cast(t, dtype):
                'float64': convert_to_float64,
                'complex64': convert_to_complex64,
                'complex128': convert_to_complex128}
+    if t.type.dtype.startswith('complex') and not dtype.startswith('complex'):
+        raise TypeError('Casting from complex to real is ambiguous: consider real(), imag(), angle() or abs()')
     return mapping[dtype](t)
 
 class Abs(UnaryScalarOp):