[clang-format] Parse the else part of #if 0

Fixes llvm#57539

Previously things outside of `#if` blocks were parsed as if only the
first branch of the conditional compilation branch existed, unless the
first condition is 0.  In that case the outer parts would be parsed as
if nothing inside the conditional parts existed.  Now we use the second
conditional branch if the first condition is 0.

Reviewed By: owenpan

Differential Revision: https://reviews.llvm.org/D133647

Author: eywdck2l

clang/lib/Format/UnwrappedLineParser.cpp

@@ -1124,7 +1124,9 @@ void UnwrappedLineParser::conditionalCompilationStart(bool Unreachable) {
   ++PPBranchLevel;
   assert(PPBranchLevel >= 0 && PPBranchLevel <= (int)PPLevelBranchIndex.size());
   if (PPBranchLevel == (int)PPLevelBranchIndex.size()) {
-    PPLevelBranchIndex.push_back(0);
+    // 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);
     PPLevelBranchCount.push_back(0);
   }
   PPChainBranchIndex.push(0);

clang/unittests/Format/FormatTest.cpp

@@ -5993,6 +5993,16 @@ TEST_F(FormatTest, LayoutStatementsAroundPreprocessorDirectives) {
                          ");\n"
                          "#else\n"
                          "#endif");
+
+  // Verify that indentation is correct when there is an `#if 0` with an
+  // `#else`.
+  verifyFormat("#if 0\n"
+               "{\n"
+               "#else\n"
+               "{\n"
+               "#endif\n"
+               "  x;\n"
+               "}");
 }
 
 TEST_F(FormatTest, GraciouslyHandleIncorrectPreprocessorConditions) {
@@ -25367,27 +25377,12 @@ TEST_F(FormatTest, InsertBraces) {
 
   verifyFormat("do {\n"
                "#if 0\n"
-               " if (a) {\n"
-               "#else\n"
-               "  if (b) {\n"
-               "#endif\n"
-               "}\n"
-               "}\n"
-               "while (0)\n"
-               "  ;",
-               Style);
-  // TODO: Replace the test above with the one below after #57539 is fixed.
-#if 0
-  verifyFormat("do {\n"
-               "#if 0\n"
-               "  if (a) {\n"
                "#else\n"
                "  if (b) {\n"
                "#endif\n"
                "  }\n"
                "} while (0);",
                Style);
-#endif
 
   Style.ColumnLimit = 15;