[libcxx-commits] [libcxx] [libc++] tests with picolibc: Fix iterator diff_type to std::streamoff (PR #74072)

[Dominik Wójt via libcxx-commits]

domin144 wrote:

@ldionne @zoecarver 

The proposed check is no longer libc++ specific. It follows this derivation:
```
std::iterator_traits<std::istreambuf_iterator<char>>::difference_type
    = std::istreambuf_iterator<char>::difference_type
    = typename Traits::off_type (Traits = std::char_traits<char>)
    = std::char_traits<char>::off_type
    = std::streamoff
```
Further is libc++ specific. `std::streamoff` is defined in `libcxx/include/__fwd/ios.h`
```
    #if defined(_NEWLIB_VERSION)
    // On newlib, off_t is 'long int'
    using streamoff = long int; // for char_traits in <string>
    #else
    using streamoff = long long; // for char_traits in <string>
    #endif
```
As an alternative, the ifdefs from `ios.h` could be reproduced in the test, making it libc++ specific again.

https://github.com/llvm/llvm-project/pull/74072