diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -845,7 +845,6 @@
       break;
     case tok::kw_if:
     case tok::kw_while:
-      assert(!Line.startsWith(tok::hash));
       if (Tok->is(tok::kw_if) && CurrentToken &&
           CurrentToken->isOneOf(tok::kw_constexpr, tok::identifier))
         next();
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -14723,6 +14723,12 @@
   verifyFormat("auto operator delete(int &) final");
 }
 
-} // end namespace
-} // end namespace format
-} // end namespace clang
+TEST_F(FormatTest, STLWhileNotDefineChed) {
+  verifyFormat("#if defined(while)\n"
+               "#define while EMIT WARNING C4005\n"
+               "#endif // while");
+}
+
+} // namespace
+} // namespace format
+} // namespace clang