Make add_control identify known definitions by type not name

[dime10]

Information

• Qiskit Terra version: 0.11.0.dev0+b7d6a90
• Python version: 3.6.9
• Operating system: Linux

What is the current behavior?

Currently when controlling a gate via gate.control(), the add_control function will check if the to be generated gate exists as special gate in extensions (such as cnot, toffoli, ect...), and create that instead of a generic one. This check is done by comparing the gate.name attribute against 'x', 'y', 'z' and so on. This can be problematic and should be replaced with a type check.

This can actually lead to a bug, for example if a gate is created from a circuit object via qc.to_gate(), and the circuit was unfortunately named (e.g. 'x'). Then the resulting gate will also have the same name, and could thus be transformed into the wrong gate upon controlling it (e.g. into a CnotGate if named 'x').

Steps to reproduce the problem

from qiskit import QuantumCircuit
qc = QuantumCircuit(3, name='x')
qc.h([0,1,2])
gate = qc.to_gate()
gate.control()

What is the expected behavior?

Suggested solutions

[willhbang]

I can make these changes if no one's started already

[dime10]

I can make these changes if no one's started already

Sounds good, I don't know of anyone who has.

[kdk]

FYI, #3631 partially addresses this problem (at least for the standard gates). Implementation on this may want to wait until that merges.

[jakelishman]

I'm going to close this as stale now, in part because the data model of Terra is that the gate name is treated as an opcode; a circuit named x must faithfully implement the Pauli X operation, and it would be a user error if it doesn't. As some other examples of constructs specifically built around the data model, we'd see the basis_gates argument to the transpiler, the required format of backend devices, and the modern Target.

Please feel free to re-open if there's more to discuss.