[libc++] Fix broken precondition of __bit_log2 (#155476)

In #135303, we started using `__bit_log2` instead of `__log2i` inside
`std::sort`. However, `__bit_log2` has a precondition that `__log2i`
didn't have, which is that the input is non-zero. While it technically
makes no sense to request the logarithm of 0, `__log2i` handled that
case and returned 0 without issues.

After switching to `__bit_log2`, passing 0 as an input results in an
unsigned integer overflow which can trigger
`-fsanitize=unsigned-integer-overflow`. While not technically UB in
itself, it's clearly not intended either.

To fix this, we add an internal assertion to `__bit_log2` which catches
the issue in our test suite, and we make sure not to violate
`__bit_log2`'s preconditions before we call it from `std::sort`.

0 files changed, 0 insertions, 0 deletions