[clang] Disallow btf_type_tag in C++ mode (PR #107238)

[Aaron Ballman via cfe-commits]

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@@ -6490,6 +6490,15 @@ static void HandleBTFTypeTagAttribute(QualType &Type, const ParsedAttr &Attr,
                                       TypeProcessingState &State) {
   Sema &S = State.getSema();
 
+  // This attribute is only supported in C.
+  // FIXME: we should implement checkCommonAttributeFeatures() in SemaAttr.cpp
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AaronBallman wrote:

Oh! It turns out `lifetimebound` is not really possible to run into in C because the only way it's accepted in a type position is as a member function qualifier. It also has some further issues in C++: https://godbolt.org/z/czPa3x7nb

So I think we should file an issue for the confusion and not make changes in this PR for it. WDYT?

https://github.com/llvm/llvm-project/pull/107238