Diagnose vertical whitespaces in single line comments

[cor3ntin]

The Unicode consortium and the C++ committee are interested in diagnosing Vertical whitespaces (VT, FF) that appear in single-line comments. We should explore a warning to guide this effort.

The motivation for this warning is that if an IDE treats such a vertical character as whitespace (which they are recommended to do), the divergence between the behavior of compilers and IDEs can lead to code spoofing, and therefore security concerns

https://www.unicode.org/reports/tr55/#Spoofing-LB
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2843r2.pdf

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Corentin Jabot (cor3ntin)

The Unicode consortium and the C++ committee are interested in diagnosing Vertical whitespaces (VT, FF) that appear in single-line comments. We should explore a warning to guide this effort.

The motivation for this warning is that if an IDE treats such a vertical character as whitespace (which they are recommended to do), the divergence between the behavior of compilers and IDEs can lead to code spoofing, and therefore security concerns

https://www.unicode.org/reports/tr55/#Spoofing-LB
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2843r2.pdf

[cor3ntin]

@tahonermann