Index: clang/lib/Lex/PPDirectives.cpp
===================================================================
--- clang/lib/Lex/PPDirectives.cpp
+++ clang/lib/Lex/PPDirectives.cpp
@@ -438,6 +438,7 @@
   }
   SourceLocation endLoc;
   while (true) {
+    assert(CurLexer);
     CurLexer->Lex(Tok);
 
     if (Tok.is(tok::code_completion)) {
@@ -1381,6 +1382,8 @@
 ///
 void Preprocessor::HandleUserDiagnosticDirective(Token &Tok,
                                                  bool isWarning) {
+  assert(CurLexer);
+
   // Read the rest of the line raw.  We do this because we don't want macros
   // to be expanded and we don't require that the tokens be valid preprocessing
   // tokens.  For example, this is allowed: "#warning `   'foo".  GCC does
@@ -1717,6 +1720,7 @@
            "This should be an early exit only to a fatal error");
     TheModuleLoader.HadFatalFailure = true;
     IncludeTok.setKind(tok::eof);
+    assert(CurLexer);
     CurLexer->cutOffLexing();
     return;
   }
@@ -2533,6 +2537,7 @@
   // Ensure we consume the rest of the macro body if errors occur.
   auto _ = llvm::make_scope_exit([&]() {
     // The flag indicates if we are still waiting for 'eod'.
+    assert(CurLexer);
     if (CurLexer->ParsingPreprocessorDirective)
       DiscardUntilEndOfDirective();
   });
Index: clang/lib/Lex/PPLexerChange.cpp
===================================================================
--- clang/lib/Lex/PPLexerChange.cpp
+++ clang/lib/Lex/PPLexerChange.cpp
@@ -317,6 +317,7 @@
     Module *M = LeaveSubmodule(/*ForPragma*/true);
 
     Result.startToken();
+    assert(CurLexer && "Got EOF but no current lexer set!");
     const char *EndPos = getCurLexerEndPos();
     CurLexer->BufferPtr = EndPos;
     CurLexer->FormTokenWithChars(Result, EndPos, tok::annot_module_end);
Index: clang/lib/Lex/Preprocessor.cpp
===================================================================
--- clang/lib/Lex/Preprocessor.cpp
+++ clang/lib/Lex/Preprocessor.cpp
@@ -637,9 +637,12 @@
         (CurLexer && CurLexer->getFileID() == getPredefinesFileID());
     switch (CurLexerKind) {
     case CLK_Lexer:
+      assert(CurLexer && "CurLexer is empty while CurLexerKind is CLK_Lexer.");
       CurLexer->Lex(Tok);
      break;
     case CLK_TokenLexer:
+      assert(CurTokenLexer &&
+          "CurTokenLexer is empty while CurLexerKind is CLK_TokenLexer.");
       CurTokenLexer->Lex(Tok);
       break;
     case CLK_CachingLexer:
@@ -895,9 +898,12 @@
   do {
     switch (CurLexerKind) {
     case CLK_Lexer:
+      assert(CurLexer && "CurLexer is empty while CurLexerKind is CLK_Lexer.");
       ReturnedToken = CurLexer->Lex(Result);
       break;
     case CLK_TokenLexer:
+      assert(CurTokenLexer &&
+          "CurTokenLexer is empty while CurLexerKind is CLK_TokenLexer.");
       ReturnedToken = CurTokenLexer->Lex(Result);
       break;
     case CLK_CachingLexer:
@@ -1217,6 +1223,7 @@
       assert(TheModuleLoader.HadFatalFailure &&
              "This should be an early exit only to a fatal error");
       Result.setKind(tok::eof);
+      assert(CurLexer);
       CurLexer->cutOffLexing();
       EnterTokens(Suffix);
       return true;