A C++20 coroutines program that would not be pleasant to debug prior to some improvements I hope to make to LLVM.

repro.cpp

#include <cstdio>
#include <experimental/coroutine>

using namespace std::experimental;

struct coro {
  struct promise_type {
    coro get_return_object() {
      auto handle = coroutine_handle<promise_type>::from_promise(*this);
      return coro(handle);
    }
    suspend_never initial_suspend() { return {}; }
    suspend_never final_suspend() { return {}; }
    void return_void() {}
    void unhandled_exception() {}
  };

  coroutine_handle<promise_type> handle;
  coro(coroutine_handle<promise_type> handle)
      : handle(handle) {}
};

coro foo() {
  int i = 0;
  ++i;
  printf("%d\n", i); // 1
  // Breakpoint 1:
  //   (lldb) frame variable i
  //   (int) i = 1

  co_await suspend_always();
  int j = 0;
  ++i;
  ++j;
  printf("%d, %d\n", i, j); // 2, 1
  // Breakpoint 2:
  //   (lldb) frame variable i
  //   (int) i = <variable not available>
  //   (lldb) frame variable j
  //   (int) j = 1

  co_await suspend_always();
  ++i;
  ++j;
  printf("%d, %d\n", j, i); // 3, 2
  // Breakpoint 3:
  //   (lldb) frame variable i
  //   (int) i = <variable not available>
  //   (lldb) frame variable j
  //   (int) j = <variable not available>
}

int main() {
  auto c = foo();
  c.handle.resume();
  c.handle.resume();
}