Index: clang-tools-extra/clang-tidy/modernize/MacroToEnumCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/modernize/MacroToEnumCheck.cpp
+++ clang-tools-extra/clang-tidy/modernize/MacroToEnumCheck.cpp
@@ -193,6 +193,7 @@
   void Endif(SourceLocation Loc, SourceLocation IfLoc) override;
   void PragmaDirective(SourceLocation Loc,
                        PragmaIntroducerKind Introducer) override;
+  void invalidateExpressionNames();
 
   // After we've seen everything, issue warnings and fix-its.
   void EndOfMainFile() override;
@@ -222,6 +223,7 @@
   void conditionStart(const SourceLocation &Loc);
   void checkCondition(SourceRange ConditionRange);
   void checkName(const Token &MacroNameTok);
+  void rememberExpressionName(const Token &MacroNameTok);
   void warnMacroEnum(const EnumMacro &Macro) const;
   void fixEnumMacro(const MacroList &MacroList) const;
 
@@ -230,6 +232,7 @@
   const SourceManager &SM;
   SmallVector<MacroList> Enums;
   SmallVector<FileState> Files;
+  std::vector<std::string> ExpressionNames;
   FileState *CurrentFile = nullptr;
 };
 
@@ -284,8 +287,9 @@
 }
 
 void MacroToEnumCallbacks::checkName(const Token &MacroNameTok) {
-  StringRef Id = getTokenName(MacroNameTok);
+  rememberExpressionName(MacroNameTok);
 
+  StringRef Id = getTokenName(MacroNameTok);
   llvm::erase_if(Enums, [&Id](const MacroList &MacroList) {
     return llvm::any_of(MacroList, [&Id](const EnumMacro &Macro) {
       return getTokenName(Macro.Name) == Id;
@@ -293,6 +297,14 @@
   });
 }
 
+void MacroToEnumCallbacks::rememberExpressionName(const Token &MacroNameTok) {
+  std::string Id = getTokenName(MacroNameTok).str();
+  auto Pos = llvm::lower_bound(ExpressionNames, Id);
+  if (Pos == ExpressionNames.end() || *Pos != Id) {
+    ExpressionNames.insert(Pos, Id);
+  }
+}
+
 void MacroToEnumCallbacks::FileChanged(SourceLocation Loc,
                                        FileChangeReason Reason,
                                        SrcMgr::CharacteristicKind FileType,
@@ -384,6 +396,8 @@
 void MacroToEnumCallbacks::MacroUndefined(const Token &MacroNameTok,
                                           const MacroDefinition &MD,
                                           const MacroDirective *Undef) {
+  rememberExpressionName(MacroNameTok);
+
   auto MatchesToken = [&MacroNameTok](const EnumMacro &Macro) {
     return getTokenName(Macro.Name) == getTokenName(MacroNameTok);
   };
@@ -447,7 +461,19 @@
     CurrentFile->GuardScanner = IncludeGuard::IfGuard;
 }
 
+void MacroToEnumCallbacks::invalidateExpressionNames() {
+  for (const std::string &Id : ExpressionNames) {
+    llvm::erase_if(Enums, [Id](const MacroList &MacroList) {
+      return llvm::any_of(MacroList, [&Id](const EnumMacro &Macro) {
+        return getTokenName(Macro.Name) == Id;
+      });
+    });
+  }
+}
+
 void MacroToEnumCallbacks::EndOfMainFile() {
+  invalidateExpressionNames();
+
   for (const MacroList &MacroList : Enums) {
     if (MacroList.empty())
       continue;
Index: clang-tools-extra/test/clang-tidy/checkers/modernize-macro-to-enum.cpp
===================================================================
--- clang-tools-extra/test/clang-tidy/checkers/modernize-macro-to-enum.cpp
+++ clang-tools-extra/test/clang-tidy/checkers/modernize-macro-to-enum.cpp
@@ -181,6 +181,12 @@
 #define USE_IFDEF 1
 #define USE_IFNDEF 1
 
+// Undef'ing first and then defining later should still exclude this macro
+#undef USE_UINT64
+#define USE_UINT64 0
+#undef USE_INT64
+#define USE_INT64 0
+
 #if defined(USE_FOO) && USE_FOO
 extern void foo();
 #else
@@ -243,6 +249,27 @@
 #define IFNDEF3 3
 #endif
 
+// Macros used in conditions are invalidated, even if they look
+// like enums after they are used in conditions.
+#if DEFINED_LATER1
+#endif
+#ifdef DEFINED_LATER2
+#endif
+#ifndef DEFINED_LATER3
+#endif
+#undef DEFINED_LATER4
+#if ((defined(DEFINED_LATER5) || DEFINED_LATER6) && DEFINED_LATER7) || (DEFINED_LATER8 > 10)
+#endif
+
+#define DEFINED_LATER1 1
+#define DEFINED_LATER2 2
+#define DEFINED_LATER3 3
+#define DEFINED_LATER4 4
+#define DEFINED_LATER5 5
+#define DEFINED_LATER6 6
+#define DEFINED_LATER7 7
+#define DEFINED_LATER8 8
+
 // Sometimes an argument to ifdef can be classified as a keyword token.
 #ifdef __restrict
 #endif