[BUG] Parameter-shift rule with postselected MCMs returns wrong values

[dwierichs]

Expected behavior

Parameter-shift differentiation with postselected MCMs works or raises an informative error.

Actual behavior

It returns wrong results.

Additional information

This topic has been discovered and discussed extensively, and a fix is known in principle.
We should either implement this fix, or deactivate the differentiation of circuits with postselected MCMs by raising an error.

Source code

x = qml.numpy.array(0.512, requires_grad=True)
for diff_method in ['backprop', 'parameter-shift']:
    print(diff_method)

    @qml.qnode(qml.device("default.qubit"), diff_method=diff_method)
    def circuit(x):
        qml.Hadamard(0)
        qml.CNOT([0, 1])
        qml.RX(x, 1)
        qml.measure(1, postselect=1)
        return qml.expval(qml.Z(0))

    print(qml.grad(circuit)(x))

Tracebacks

backprop
0.48992175838037155
parameter-shift
0.0

System information

pl dev

Existing GitHub issues

• I have searched existing GitHub issues to make sure the issue does not already exist.