[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

3029

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

if (Directive.startswith("if")) {
           DiscardUntilEndOfDirective();
         } else {
           // Restore the value of LexingRawMode so that identifiers are
           // looked up, etc, inside the #elif expression.
           assert(CurPPLexer->LexingRawMode && "We have to be skipping here!");
           CurPPLexer->LexingRawMode = false;
           IdentifierInfo *IfNDefMacro = nullptr;
           DirectiveEvalResult DER = EvaluateDirectiveExpression(IfNDefMacro);
+          // Stop if Lexer became invalid after hitting code completion token.
+          if (!CurPPLexer)
+            return;
           const bool CondValue = DER.Conditional;
           CurPPLexer->LexingRawMode = true;
           if (Callbacks) {
             Callbacks->Elif(
                 Tok.getLocation(), DER.ExprRange,
                 (CondValue ? PPCallbacks::CVK_True : PPCallbacks::CVK_False),
                 CondInfo.IfLoc);
           }
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 evaluating
+  // 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
+#if FOO(
+#elif FOO(
+#endif
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:2:9 %s
+// RUN: not %clang_cc1 -fsyntax-only -code-completion-at=%s:3:11 %s