Possible test case: apply the operation to 256-bit integer 0x8000000000000000000000000000000000000000000000000000000000000000 (first bit from the left is set, all other bits cleared)
Correct answer: 0. The first bit from the left is set.
Current implementation's answer (probably, not tested yet): 128. It counts the bits cleared in the 2 blocks (64 bits each) in the middle of the number (bits 64 to 191) before finding the bit set in the left-most block (block 3).

I was playing with the core libraries and details of C++ and tried compiling and running programs without standard libraries and with custom versions of standard library.
I was implementing bit operations and decided to take a look at llvm's implementation to compare with my own. I had a very similar idea, but the rotation direction was different on libcxx.
That's how I found it. I didn't had a test case.