DiagonalGate does not raise when diagonal entries do not have modulus 1

[hhyap]

Information

• Qiskit Terra version: 0.12.0
• Python version: 3.6.9
• Operating system: Linux

What is the current behavior?

diag_gate allows entries whose modulus are not one.

Steps to reproduce the problem

import qiskit
from numpy import exp,pi

backend = qiskit.Aer.get_backend("statevector_simulator")
qc = qiskit.QuantumCircuit(1)
qc.h(0)
qc.diag_gate([exp(1j*pi/8),0],qc.qregs)  # <-- no error here
qiskit.execute(qc,backend).result().to_dict()

What is the expected behavior?

An error should be raised.

Suggested solutions

Line 59 in https://github.com/Qiskit/qiskit-terra/blob/master/qiskit/extensions/quantum_initializer/diag.py#L59:
if not np.abs( np.abs(z) - 1 ) < _EPS:

[JiahanBro]

Hey guys! I would like to give it a try. Just to be sure I understood it correctly, the second entry of the list which is 0 seems to be the problem right? Maybe the if case in Line 59 only checks the first entry, therefore no error is raised?

[Dinesh-byte-cloud]

hey can I have a shot at this

[Cryoris]

There's an old PR that should fix this, #4375. It has some merge conflicts, maybe you can check with @ewinston if you can brush it up, or create a new one based on that 🙂