Environment

• Qiskit Terra version: dd7f939
• Python version: 3.10.8
• Operating system: Arch Linux

What is happening?

Objects which compare equal must have the same hash value; see https://docs.python.org/3/reference/datamodel.html#object.hash. This property is violated by Parameter.

How can we reproduce the issue?

from qiskit.circuit import Parameter

a = Parameter("a")
aa = a + 0

print(a == aa)
print(hash(a) == hash(aa))

True
False

Note that a has type Parameter but aa has type ParameterExpression

What should happen?

Any one of the following is acceptable in theory:

1. a == aa and hash(a) == hash(aa) both evaluate to True.
2. a == aa and hash(a) == hash(aa) both evaluate to False.
3. hash(a) raises TypeError: unhashable type

Any suggestions?

Another option would be to deprecate Parameter and instead use sympy.Symbol directly.