[BUG]: unreachable code in CCCL cuda std header

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Type of Bug

Compile-time Error

### Component

libcu++

### Describe the bug

Code that has warnings on, will not compile included headers

### How to Reproduce

All the details are here: https://gitlab.com/correaa/boost-multi/-/jobs/15466817828

clang-cl.exe

### Expected behavior

Now warning should be triggered

### Reproduction link

_No response_

### Operating System

Windows 11 with clang-cl.exe v19

### nvidia-smi output

N/A

### NVCC version

N/A