Using static_cast on measurement results can cause the nvq++ compiler to crash despite the compiler knowing how to do that cast.

[bmhowe23]

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Describe the bug

Performing static_cast<bool>(measure_result) causes cudaq-quake (part of nvq++) to crash with an unhelpful error message.

Currently the problem only happens when using a target that uses MLIR mode (hardware backends, NVQC, and any time the user runs with --enable-mlir).

The problem is technically twofold:

• The compiler should accept the static_cast() operation, especially since the compiler can convert between measure_result and bool if the static_cast is not performed. (See below for example.)
• The compiler gives an unhelpful error message

Steps to reproduce the bug

The following code reproduces the problem:

#include "cudaq.h"

bool xor_kernel() __qpu__ {
  cudaq::qvector qvec(7);
  x(qvec);
  auto res = mz(qvec);
  bool xor_result = false;
  
  // This fails
  for (auto x : res)
    xor_result ^= static_cast<bool>(x);

  // This works
  // for (auto x : res)
  //   xor_result ^= x;

  return xor_result;
}

int main(int argc, char *argv[]) {
  bool result = xor_kernel();
  printf("Result = %d\n", static_cast<int>(result));
  return 0;
}

$ nvq++ --enable-mlir issue.cpp
cudaq-quake: /workspaces/cuda-quantum/lib/Frontend/nvqpp/ConvertExpr.cpp:615: bool cudaq::details::QuakeBridgeVisitor::TraverseImplicitCastExpr(clang::ImplicitCastExpr*, clang::RecursiveASTVisitor<cudaq::details::QuakeBridgeVisitor>::DataRecursionQueue*): Assertion `typeStack.size() == typeStackDepth && "must be original depth"' failed.
PLEASE submit a bug report to https://github.com/NVIDIA/cuda-quantum and include the crash backtrace.

Another example that produces the problem is:

#include "cudaq.h"

bool xor_kernel() __qpu__ {
  cudaq::qubit q;
  x(q);
  auto res = mz(q);
  bool xor_result = false;
  xor_result ^= static_cast<bool>(res); // This fails
  // xor_result ^= res;                 // This works
  return xor_result;
}

int main(int argc, char *argv[]) {
  bool result = xor_kernel();
  printf("Result = %d\n", static_cast<int>(result));
  return 0;
}

Expected behavior

The compiler should behave similar to the "This works" section of the above example. That is - it should correctly perform the static_cast operation.

Is this a regression? If it is, put the last known working version (or commit) here.

Not a regression

Environment

• CUDA Quantum version: latest (f798375)
• Python version:
• C++ compiler:
• Operating system:

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