Incorrect simulation of circuit containing initial conditional R1XY

[cqc-alec]

The PHIR pasted below expresses the following logic:

1. Initialize one qubit register "q", classical registers "a" and "t", all of size 1.
2. If t[0] == 0, apply R1XY(0.6366197723675814*pi, 0) to q[0].
3. Apply R1XY(3.3633802276324185*pi, 0) to q[0].
4. Measure q[0] to a[0].

Since t[0] == 0 initially, and 0.6366197723675814 + 3.3633802276324185 = 4, both R1XY gates should be applied and should cancel, so the final measurement in a[0] should be zero.

However, when run 100 times with the statevector simulator the result is a mixture of 0s and 1s.

The angles used here are (in radians) 2 and 4*pi - 2. When 1.9 and 4*pi - 1.9 are used, the results are as expected (all zero).

from pecos.engines.hybrid_engine import HybridEngine

def test():
    phir = """
{
    "format": "PHIR/JSON",
    "version": "0.1.0",
    "metadata":
    {
        "source": "pytket-phir v0.8.1"
    },
    "ops":
    [
        {
            "data": "qvar_define",
            "data_type": "qubits",
            "variable": "q",
            "size": 1
        },
        {
            "data": "cvar_define",
            "data_type": "i64",
            "variable": "a",
            "size": 1
        },
        {
            "data": "cvar_define",
            "data_type": "i64",
            "variable": "t",
            "size": 1
        },
        {
            "block": "if",
            "condition":
            {
                "cop": "==",
                "args":
                [
                    [
                        "t",
                        0
                    ],
                    0
                ]
            },
            "true_branch":
            [
                {
                    "qop": "R1XY",
                    "angles":
                    [
                        [
                            0.6366197723675814,
                            0.0
                        ],
                        "pi"
                    ],
                    "args":
                    [
                        [
                            "q",
                            0
                        ]
                    ]
                }
            ]
        },
        {
            "qop": "R1XY",
            "angles":
            [
                [
                    3.3633802276324185,
                    0.0
                ],
                "pi"
            ],
            "args":
            [
                [
                    "q",
                    0
                ]
            ]
        },
        {
            "qop": "Measure",
            "returns":
            [
                [
                    "a",
                    0
                ]
            ],
            "args":
            [
                [
                    "q",
                    0
                ]
            ]
        }
    ]
}
"""
    engine = HybridEngine(qsim="state-vector")
    results = engine.run(phir, shots=100)
    print(results)