[Miscompile] Incorrect defunctionalization #3768

"""
`Choose(true)` should return `H`, so the QIR should apply `h` to qubit 0. The
defunctionalized branch tree remaps both `b` guards to the same caller expression
and keeps the last inner default, so the QIR applies `x` instead. There's no panic.

Exits 0 while the miscompile reproduces. Found by opus-b17 in the panic hunt
(qsc_fir_transforms/src/defunctionalize/rewrite.rs, build_branch_tree).
"""

import re
import sys

from qdk import qsharp

SOURCE = """
namespace Demo {
    function Choose(b : Bool) : (Qubit => Unit) {
        if b { H } else { if b { X } else { Z } }
    }
    operation Main() : Result {
        use q = Qubit();
        let arr = [1, 2, 3];
        let x = Length(arr) > 1;
        let f = Choose(x);
        f(q);
        MResetZ(q)
    }
}
"""

qsharp.init(target_profile=qsharp.TargetProfile.Adaptive_RIF)
qsharp.eval(SOURCE)
qir = str(qsharp.compile("Demo.Main()"))
gates = re.findall(r"@__quantum__qis__(\w+)__body\(%Qubit\* (?:null|inttoptr \(i64 0 to %Qubit\*\))", qir)
print("gates on qubit 0:", gates)

if "x" in gates and "h" not in gates:
    print("REPRODUCED: QIR applies X where Choose(true) should give H")
    sys.exit(0)
print("NOT REPRODUCED")
sys.exit(1)