[llvm-bugs] [Bug 50203] New: Incorrect code gen related to std::variant

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50203

            Bug ID: 50203
           Summary: Incorrect code gen related to std::variant
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: CUDA
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ryan_greenblatt at brown.edu
                CC: llvm-bugs at lists.llvm.org

```
#include <thrust/execution_policy.h>
#include <thrust/for_each.h>
#include <thrust/iterator/counting_iterator.h>

#include <variant>

struct Items {
  bool needed;
  std::array<unsigned, 2> also_needed;
};

struct Value {
  std::variant<unsigned, Items> item;

  template <typename F> constexpr void get_first(F &&f) {
    return f(*std::get_if<0>(&item));
  }
};

constexpr Value get_variant() { return {.item = 0u}; }

int main() {
  auto it = thrust::make_counting_iterator(0u);
  thrust::for_each(thrust::device, it, it + 1, [](auto) {
    get_variant().get_first(
        [](const auto &v) { printf("should be zero: %u\n", v); });
  });
}
```

When built with `clang++ -O0 --cuda-gpu-arch=sm_75 --cuda-path=/opt/cuda
-std=c++17 -pthread -lcudart_static -ldl -lrt -L/opt/cuda/lib64` (modify path
as needed), this program prints out (seemingly) random values instead of always
printing zero.

This only happens when building with O0.

I tried reducing the test case further, but I was unable to do so (launching
the kernel without using thrust seems to remove the issue).

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