[clang-format] Fix for #58188

[sstwcw]

Summary

This is intended to provide a hint for how to fix the bug. The patch fixes the bug itself, but a problem remains.

[sstwcw]

See https://github.com/llvm/llvm-project/issues/58188#issuecomment-1270355888.

[HazardyKnusperkeks]

What is the problem that remains?

Please add a test case.

clang/lib/Format/UnwrappedLineParser.cpp

1142–1143

Could you remove the negation? I find negations of compound statements always harder to read.

[sstwcw]

The problem that remains is this, as I explained in the issue. The second branch of the second block is not indented.

#if 1
{
#endif
#if X
  x;
#else
x;
#endif
}

I didn't add a test case because I was not expecting this patch to be committed as there is still a problem.

[sstwcw]

I am closing it now there is D135740.

clang/lib/Format/UnwrappedLineParser.cpp

if (Unreachable ||
     PPStack.push_back({PP_Conditional, Line});
   }
 }
 
 void UnwrappedLineParser::conditionalCompilationStart(bool Unreachable) {
   ++PPBranchLevel;
   assert(PPBranchLevel >= 0 && PPBranchLevel <= (int)PPLevelBranchIndex.size());
   if (PPBranchLevel == (int)PPLevelBranchIndex.size()) {
-    // If the first branch is unreachable, set the BranchIndex to 1.  This way
-    // the next branch will be parsed if there is one.
-    PPLevelBranchIndex.push_back(Unreachable ? 1 : 0);
+    PPLevelBranchIndex.push_back(0);
     PPLevelBranchCount.push_back(0);
   }
   PPChainBranchIndex.push(0);
   bool Skip = PPLevelBranchIndex[PPBranchLevel] > 0;
   conditionalCompilationCondition(Unreachable || Skip);
 }
 
 void UnwrappedLineParser::conditionalCompilationAlternative() {
+  bool FirstBranchSkipped =
+      !PPStack.empty() && PPLevelBranchIndex[PPBranchLevel] == 0 &&
+      PPStack.back().Kind == PP_Unreachable &&
+      !(PPStack.size() > 1 &&
+        PPStack[PPStack.size() - 2].Kind == PP_Unreachable);
   if (!PPStack.empty())
     PPStack.pop_back();
   assert(PPBranchLevel < (int)PPLevelBranchIndex.size());
   if (!PPChainBranchIndex.empty())
     ++PPChainBranchIndex.top();
-  conditionalCompilationCondition(
-      PPBranchLevel >= 0 && !PPChainBranchIndex.empty() &&
-      PPLevelBranchIndex[PPBranchLevel] != PPChainBranchIndex.top());
+  // If the first branch is skipped because the condition is 0, parse the second
+  // branch.
+  conditionalCompilationCondition(/*Unreachable=*/
+                                  PPBranchLevel >= 0 &&
+                                  !PPChainBranchIndex.empty() &&
+                                  PPLevelBranchIndex[PPBranchLevel] !=
+                                      PPChainBranchIndex.top() &&
+                                  !FirstBranchSkipped);
 }
 
 void UnwrappedLineParser::conditionalCompilationEnd() {
   assert(PPBranchLevel < (int)PPLevelBranchIndex.size());
   if (PPBranchLevel >= 0 && !PPChainBranchIndex.empty()) {
     if (PPChainBranchIndex.top() + 1 > PPLevelBranchCount[PPBranchLevel])
       PPLevelBranchCount[PPBranchLevel] = PPChainBranchIndex.top() + 1;
   }