[clang] [clang] Support header shadowing diagnostics in Clang header search (PR #162491)

[Jinjie Huang via cfe-commits]

Jinjie-Huang wrote:

@tahonermann Thank you for review and sharing the helpful context.​

> I am worried that this diagnostic will be quite noisy for some projects. I think a more useful improvement would be more diagnostics for cases where `clang` and `clang-cl` would resolve a header file differently. I see you already found the one existing case of such a diagnostic [here](https://github.com/llvm/llvm-project/blob/e29d4ebb89705162b588caf69b45581f1da3a45c/clang/lib/Lex/HeaderSearch.cpp#L1000-L1012).

Yes, I agree that it could potentially generate noise, especially if a project intentionally uses header files with the same name and carefully designs the -Ioptions. 
The consideration here is: currently, this diagnostic is disabled by default. Perhaps we could enable it in debugging scenarios, such as when sanitizers are active, to help users discover subtle API mismatch problems. Alternatively, it could be grouped and enabled alongside something like the[ "ShadowAll DiagGroup"](https://github.com/llvm/llvm-project/blob/main/clang/include/clang/Basic/DiagnosticGroups.td#L797), or we could consider enabling it by default at a more appropriate time in the future? After all, having numerous header files with the same name in a project does not seem to be a recommended practice, it could indeed lead to some difficult-to-trace problems.


> Is there special handling for `#include_next`? If not, wouldn't effectively every use of `#include_next` be accompanied by this new warning? Or am I misunderstanding something?

According to my observations, not every #include_nextis accompanied by this new warning. Let me explain how it works (please correct me if I'm wrong):

The MSVC rule does not support 'include_next', as you mentioned, so the search primarily relies on the regular '-I' strategy. The current lookup is implemented using an iterator ([code](https://github.com/llvm/llvm-project/blob/main/clang/lib/Lex/HeaderSearch.cpp#L1021-L1027)). When Include_nextcalls HeaderSearch::LookupFile, if FromDiris not empty, the iterator is set to start from the position right after the previously searched path, thereby avoiding finding the same file again.

Thus, include_next is naturally compatible with this mechanism (thanks to the baseline implementation). Unless there is a header file with the same name in subsequent search paths, no warning will be triggered—and if one is triggered, that is actually the expected behavior.


https://github.com/llvm/llvm-project/pull/162491