[libcxx-commits] [libcxx] [libc++] Use static_asserts for span::front() and span::back() when possible (PR #119381)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 requested changes to this pull request.

Is this allowed? You could have code like
```
if (!span.empty()) { span.front(); }

```
which doesn't violate the precondition, but would be rejected with this patch.

https://github.com/llvm/llvm-project/pull/119381