[SourceManager] Explicitly check for potential iterator underflow

[jkorous]

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[arphaman]

Do you have a test case? Please add a comment stating why the check is needed.

[jkorous]

Unfortunately, I don't have a test, sorry. I'm not trying to cut corners here - just thought that iterator referring to begin() value of a container should never be decremented so given the nature of the issue, simplicity of the fix and expected effort necessary to find a reliable test case it'd be more productive if I spend my time doing other work.

But please tell me if you think this is not an obvious bug and an obvious way to fix it - I could be missing something.

[jkorous]

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[arphaman]

Thanks!

clang/lib/Basic/SourceManager.cpp

SourceManager::getMacroArgExpandedLocation(SourceLocation Loc) const {
   std::unique_ptr<MacroArgsMap> &MacroArgsCache = MacroArgsCacheMap[FID];
   if (!MacroArgsCache) {
     MacroArgsCache = std::make_unique<MacroArgsMap>();
     computeMacroArgsCache(*MacroArgsCache, FID);
   }
 
   assert(!MacroArgsCache->empty());
   MacroArgsMap::iterator I = MacroArgsCache->upper_bound(Offset);
+  // In case every element in MacroArgsCache is greater than Offset we can't
+  // decrement the iterator.
+  if (I == MacroArgsCache->begin())
+    return Loc;
+
   --I;
 
   unsigned MacroArgBeginOffs = I->first;
   SourceLocation MacroArgExpandedLoc = I->second;
   if (MacroArgExpandedLoc.isValid())
     return MacroArgExpandedLoc.getLocWithOffset(Offset - MacroArgBeginOffs);
 
   return Loc;