Index: clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.h
===================================================================
--- clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.h
+++ clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.h
@@ -1,4 +1,4 @@
-//===--- SpecialMemberFunctionsCheck.h - clang-tidy-------------------*- C++ -*-===//
+//===--- SpecialMemberFunctionsCheck.h - clang-tidy--------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -30,7 +30,7 @@
   void registerMatchers(ast_matchers::MatchFinder *Finder) override;
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
   void onEndOfTranslationUnit() override;
-  
+
   enum class SpecialMemberFunctionKind {
     Destructor,
     CopyConstructor,
@@ -41,15 +41,11 @@
 
   using ClassDefId = std::pair<SourceLocation, std::string>;
 
-  using ClassDefiningSpecialMembersMap = llvm::DenseMap<ClassDefId, llvm::SmallVector<SpecialMemberFunctionKind, 5>>;
+  using ClassDefiningSpecialMembersMap =
+      llvm::DenseMap<ClassDefId,
+                     llvm::SmallSetVector<SpecialMemberFunctionKind, 5>>;
 
 private:
-  
-  static llvm::StringRef toString(SpecialMemberFunctionKind K);
-
-  static std::string join(llvm::ArrayRef<SpecialMemberFunctionKind> SMFS,
-                          llvm::StringRef AndOr);
-  
   ClassDefiningSpecialMembersMap ClassWithSpecialMembers;
 };
 
@@ -60,12 +56,12 @@
 namespace llvm {
 /// Specialisation of DenseMapInfo to allow ClassDefId objects in DenseMaps
 /// FIXME: Move this to the corresponding cpp file as is done for
-/// clang-tidy/readability/IdentifierNamingCheck.cpp. 
+/// clang-tidy/readability/IdentifierNamingCheck.cpp.
 template <>
 struct DenseMapInfo<
     clang::tidy::cppcoreguidelines::SpecialMemberFunctionsCheck::ClassDefId> {
   using ClassDefId =
-    clang::tidy::cppcoreguidelines::SpecialMemberFunctionsCheck::ClassDefId;
+      clang::tidy::cppcoreguidelines::SpecialMemberFunctionsCheck::ClassDefId;
 
   static inline ClassDefId getEmptyKey() {
     return ClassDefId(
Index: clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.cpp
===================================================================
--- clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.cpp
+++ clang-tidy/cppcoreguidelines/SpecialMemberFunctionsCheck.cpp
@@ -43,25 +43,25 @@
       this);
 }
 
-llvm::StringRef SpecialMemberFunctionsCheck::toString(
-    SpecialMemberFunctionsCheck::SpecialMemberFunctionKind K) {
+static llvm::StringRef
+toString(SpecialMemberFunctionsCheck::SpecialMemberFunctionKind K) {
   switch (K) {
-  case SpecialMemberFunctionKind::Destructor:
+  case SpecialMemberFunctionsCheck::SpecialMemberFunctionKind::Destructor:
     return "a destructor";
-  case SpecialMemberFunctionKind::CopyConstructor:
+  case SpecialMemberFunctionsCheck::SpecialMemberFunctionKind::CopyConstructor:
     return "a copy constructor";
-  case SpecialMemberFunctionKind::CopyAssignment:
+  case SpecialMemberFunctionsCheck::SpecialMemberFunctionKind::CopyAssignment:
     return "a copy assignment operator";
-  case SpecialMemberFunctionKind::MoveConstructor:
+  case SpecialMemberFunctionsCheck::SpecialMemberFunctionKind::MoveConstructor:
     return "a move constructor";
-  case SpecialMemberFunctionKind::MoveAssignment:
+  case SpecialMemberFunctionsCheck::SpecialMemberFunctionKind::MoveAssignment:
     return "a move assignment operator";
   }
   llvm_unreachable("Unhandled SpecialMemberFunctionKind");
 }
 
-std::string SpecialMemberFunctionsCheck::join(
-    llvm::ArrayRef<SpecialMemberFunctionKind> SMFS, llvm::StringRef AndOr) {
+template <typename R>
+static std::string join(const R &SMFS, llvm::StringRef AndOr) {
 
   assert(!SMFS.empty() &&
          "List of defined or undefined members should never be empty.");
@@ -97,7 +97,7 @@
 
   for (const auto &KV : Matchers)
     if (Result.Nodes.getNodeAs<CXXMethodDecl>(KV.first))
-      ClassWithSpecialMembers[ID].push_back(KV.second);
+      ClassWithSpecialMembers[ID].insert(KV.second);
 }
 
 void SpecialMemberFunctionsCheck::onEndOfTranslationUnit() {
@@ -112,7 +112,7 @@
   }
 
   for (const auto &C : ClassWithSpecialMembers) {
-    ArrayRef<SpecialMemberFunctionKind> DefinedSpecialMembers = C.second;
+    const auto &DefinedSpecialMembers = C.second;
 
     if (DefinedSpecialMembers.size() == AllSpecialMembers.size())
       continue;