[PATCH] D156056: [clang-tidy] Initialize DiagnosticEngine in ExpandModularHeaders

[Carlos Galvez via Phabricator via cfe-commits]

carlosgalvezp added a comment.

A thought came to mind - since we are doing workarounds anyway, would it be easier to ask people to simply add `-clang-diagnostic*` to the `Checks` in their config file? It's fair to assume they will get those warnings when compiling the code. I feel the more workarounds we add in the code the harder it will be to clean it up later :)



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Comment at: clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp:75
+      Diags(new DiagnosticIDs,
+            new DiagnosticOptions(Compiler.getDiagnosticOpts()),
             new ForwardingDiagnosticConsumer(Compiler.getDiagnosticClient())),
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When downloading your patch, this seems to not be needed to make the tests pass, should it be removed?


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Comment at: clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp:83
   Diags.setSourceManager(&Sources);
+  ProcessWarningOptions(Diags, Compiler.getDiagnosticOpts());
 
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PiotrZSL wrote:
> carlosgalvezp wrote:
> > A bit unclear to me why we should add this line here, grepping for this function in the repo I only find hits in the `clang` folder. How come it's not needed in other places?
> We create here new Preprocessor (line 96) and new DiagEngine (line 74), when C++20/Modules are enabled this class is register as an second Preprocessor and both are (+-) executed.
> Unfortunately when we pass `-Wno-macro-redefined` it's pass only to original DiagEngine, and we run into situation when warning is suppressed by first DiagEngine, but not by second that is used by second Preprocessor. 
> 
> Passing DiagnosticOptions alone to DiagEngine looks to be insufficient, as it's does not apply settings, only calling this function apply them. (somehow).
> This is gray area for me.
> 
> More about problem here: https://discourse.llvm.org/t/rfc-expand-modular-headers-ppcallbacks-problem-in-c-20/71628
Thanks for the explanation! I'm not sure what the best way forward is. Would it make sense to add some `TODO` or `FIXME` comment to further investigate in the future if we want that line of code ?


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