[clang][PreProcessor] Cutoff parsing after hitting completion point

[kadircet]

Summary

This fixes a crash caused by Lexers being invalidated at code
completion points in
https://github.com/llvm/llvm-project/blob/main/clang/lib/Lex/PPLexerChange.cpp#L520.

[hokein]

clang/lib/Lex/PPDirectives.cpp

3026

evulating => evaluating

IIUC, CurPPLexer may be reset to nullptr after the above EvaluateDirectiveExpression call? If so, I think there is another place having the same pattern, line 597 of this file.

[kadircet]

• Fix typo in comment
• Handle the \#elif case too.

[kadircet]

• Fix lit

[hokein]

thanks!

clang/lib/Lex/PPDirectives.cpp

void Preprocessor::HandleIfDirective(Token &IfToken,
                                      const Token &HashToken,
                                      bool ReadAnyTokensBeforeDirective) {
   ++NumIf;
 
   // Parse and evaluate the conditional expression.
   IdentifierInfo *IfNDefMacro = nullptr;
   const DirectiveEvalResult DER = EvaluateDirectiveExpression(IfNDefMacro);
   const bool ConditionalTrue = DER.Conditional;
+  // Lexer might become invalid if we hit code completion point while evulating
+  // expression.
+  if (!CurPPLexer)
+    return;
 
   // If this condition is equivalent to #ifndef X, and if this is the first
   // directive seen, handle it for the multiple-include optimization.
   if (CurPPLexer->getConditionalStackDepth() == 0) {
     if (!ReadAnyTokensBeforeDirective && IfNDefMacro && ConditionalTrue)
       // FIXME: Pass in the location of the macro name, not the 'if' token.
       CurPPLexer->MIOpt.EnterTopLevelIfndef(IfNDefMacro, IfToken.getLocation());
     else

clang/test/CodeCompletion/crash-if-directive.cpp

+#define FOO(X) X + 1
+#if FOO(
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:2:9 %s | FileCheck %s --allow-empty