Parameter substitution for xacc::CompositeInstruction using operator() is inconsistent

[daanish-qb]

When using a parametrized xacc::CompositeInstruction created using the IR, the values for variables that are fed in using the () operator (or by using ->operator()(params)) are not substituted consistently, depending only on the name of the variable. For example, for the following circuit:

Rx(alpha[0]) q0
Ry(alpha[0]) q0
Rx(beta[0]) q1
U1(beta[0]) q1
Measure q0
Measure q1

When the following code is run:

auto vars = instructions->getVariables();
  std::cout << "Variables: ";
  for (auto& elem: vars) {
    std::cout <<  elem << ",";
  }
  std::cout << "\n\n";
  std::vector<double> param_vec(2);
  param_vec[0] = M_PI_2;
  param_vec[1] = M_PI_4;
  std::cout << "Parameter vector: [";
  for (auto&  elem: param_vec) {
    std::cout << elem << ",";
  }
  std::cout << "]\n\n";
  std::shared_ptr<xacc::CompositeInstruction> evaled_circ =
      (*instructions)(param_vec);
  std::cout << evaled_circ->toString() << "\n";

This is the resultant output:

Variables: beta[0],alpha[0],

Parameter vector: [1.5708,0.785398,]

Rx(6.93907e-310) q0
Ry(6.93907e-310) q0
Rx(6.93907e-310) q1
U1(6.93907e-310) q1
Measure q0
Measure q1

Meaning the CompositeInstruction recognizes the variables, but instead keeps the values of the parameters as 0. Instead, if the names of the variables are changed to simply be "alpha" and "beta", resulting in the following circuit:

Rx(alpha) q0
Ry(alpha) q0
Rx(beta) q1
U1(beta) q1
Measure q0
Measure q1

The above code runs as expected:

Variables: beta,alpha,

Parameter vector: [1.5708,0.785398,]

Rx(1.5708) q0
Ry(1.5708) q0
Rx(0.785398) q1
U1(0.785398) q1
Measure q0
Measure q1

This is an interesting issue, and definitely one that presents a problem if trying to build up separate vectors of parameters. Any insight would be appreciated.