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

[sstwcw]

Summary

Fixes https://github.com/llvm/llvm-project/issues/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, then the outer parts would be parsed as if
anything inside the conditional parts didn't exist. Now we use the
second conditional branch if the first condition is 0.

[owenpan]

Thanks!

clang/lib/Format/UnwrappedLineParser.cpp

1128

[sammccall]

This seems to have regressed comment alignment on unrelated directives:

#if 0
#endif

#if X
int something_fairly_long;  // Align here please
#endif                      // Should be aligned

These comments were aligned before this patch, but are no longer.
I'm not sure why, maybe setting the taken branch to 1 has some negative side-effect when there's no branch 1?
Filed https://github.com/llvm/llvm-project/issues/58188

[owenpan]

See D137052.

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()) {
-    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);
   bool Skip = PPLevelBranchIndex[PPBranchLevel] > 0;
   conditionalCompilationCondition(Unreachable || Skip);
 }
 
 void UnwrappedLineParser::conditionalCompilationAlternative() {

clang/unittests/Format/FormatTest.cpp

verifyFormat("DEBUG({\n"
                "#else\n"
                "#endif");
 
   verifyIncompleteFormat("void f(\n"
                          "#if A\n"
                          ");\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) {
   verifyFormat("#endif\n"
                "#if B");
 }
 
 TEST_F(FormatTest, FormatsJoinedLinesOnSubsequentRuns) {
verifyFormat("#if 0\n"
                "#endif\n"
                "void f() {\n"
                "  if (a) g();\n"
                "}",
                Style);
 
   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;
 
   verifyFormat("#define A     \\\n"
                "  if (a)      \\\n"
                "    f();",
                Style);