`[[nodiscard]]` should be applied to functions where discarding the return value is most likely a correctness issue. - Applied to (if applicable): - [x] `basic_streambuf` - [x] `basic_istream` - [x] `basic_ostream` - [x] `basic_iostream` (none) - https://libcxx.llvm.org/CodingGuidelines.html - https://wg21.link/stream.buffers - https://wg21.link/iostreamclass - https://wg21.link/istream - https://wg21.link/ostream Towards #172124