[clang-tidy]: modernize-use-using: don't diagnose typedefs in `extern "C"` DeclContexts

[george.burgess.iv]

Summary

If code is shared between C and C++, converting a typedef to a using isn't possible. Being more conservative about emitting these lints in extern "C" blocks seems like a good compromise to me.

[Eugene.Zelenko]

See also https://llvm.org/PR35924.

[george.burgess.iv]

I see -- that seems like a much broader change, but also probably worthwhile. I have a few ideas about how to tackle that; let me see what I can come up with locally.

Thanks!

clang-tools-extra/clang-tidy/modernize/UseUsingCheck.cpp

void UseUsingCheck::check(const MatchFinder::MatchResult &Result) {
   if (MatchedDecl->getLocation().isInvalid())
     return;
 
   SourceLocation StartLoc = MatchedDecl->getBeginLoc();
 
   if (StartLoc.isMacroID() && IgnoreMacros)
     return;
 
+  if (MatchedDecl->getDeclContext()->isExternCContext())
+    return;
+
   static const char *UseUsingWarning = "use 'using' instead of 'typedef'";
 
   // Warn at StartLoc but do not fix if there is macro or array.
   if (MatchedDecl->getUnderlyingType()->isArrayType() || StartLoc.isMacroID()) {
     diag(StartLoc, UseUsingWarning);
     return;
   }
 

clang-tools-extra/test/clang-tidy/checkers/modernize-use-using.cpp

 // CHECK-FIXES: using EnumT1 = enum { ea1, eb1 };
 
 #include "modernize-use-using.h"
 
 typedef enum { ea2, eb2 } EnumT2_CheckTypedefImpactFromAnotherFile;
 // CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use 'using' instead of 'typedef'
 // CHECK-FIXES: using EnumT2_CheckTypedefImpactFromAnotherFile = enum { ea2, eb2 };
 
+extern "C" {
+typedef int Type;
+// No messages expected, since this code may be shared verbatim with C.
+}