[llvm-branch-commits] [clang] release/21.x: [clang] Allow trivial pp-directives before C++ module directive (#153641) (PR #154077)

[Boris Kolpackov via llvm-branch-commits]

boris-kolpackov wrote:

@erichkeane, yes, as @yronglin mentioned, this bug makes it impossible to compile a module translation unit that was partially preprocessed with Clang's `-frewrite-includes` (which is what we do in `build2`). This was not an issue up to Clang 20, so seems like a regression to me. It is affecting `build2`'s users, see this report for an example: https://github.com/llvm/llvm-project/issues/151985

https://github.com/llvm/llvm-project/pull/154077