diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -1127,9 +1127,7 @@
   ++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);
@@ -1138,14 +1136,24 @@
 }
 
 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() {