Index: clang/lib/Format/UnwrappedLineParser.cpp
===================================================================
--- clang/lib/Format/UnwrappedLineParser.cpp
+++ clang/lib/Format/UnwrappedLineParser.cpp
@@ -1144,12 +1144,11 @@
   ++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);
+  if (!Unreachable)
+    PPChainBranchIndex.push(0);
   bool Skip = PPLevelBranchIndex[PPBranchLevel] > 0;
   conditionalCompilationCondition(Unreachable || Skip);
 }
Index: clang/unittests/Format/FormatTest.cpp
===================================================================
--- clang/unittests/Format/FormatTest.cpp
+++ clang/unittests/Format/FormatTest.cpp
@@ -6119,7 +6119,7 @@
                          "#endif");
 
   // Verify that indentation is correct when there is an `#if 0` with an
-  // `#else`.
+  // `#else`/`#elif`.
   verifyFormat("#if 0\n"
                "{\n"
                "#else\n"
@@ -6127,6 +6127,26 @@
                "#endif\n"
                "  x;\n"
                "}");
+  verifyFormat("#if 0\n"
+               "{\n"
+               "#elif FOO\n"
+               "{\n"
+               "#endif\n"
+               "  x;\n"
+               "}");
+
+  verifyFormat("#if 0\n"
+               "#endif\n"
+               "#if X\n"
+               "int something_fairly_long; // Align here please\n"
+               "#endif                     // Should be aligned");
+
+  verifyFormat("#if 0\n"
+               "#endif\n"
+               "#if X\n"
+               "#else  // Align\n"
+               ";\n"
+               "#endif // Align");
 }
 
 TEST_F(FormatTest, GraciouslyHandleIncorrectPreprocessorConditions) {