Index: clang-tidy/misc/CMakeLists.txt
===================================================================
--- clang-tidy/misc/CMakeLists.txt
+++ clang-tidy/misc/CMakeLists.txt
@@ -6,12 +6,13 @@
   AssignOperatorSignatureCheck.cpp
   BoolPointerImplicitConversionCheck.cpp
   DefinitionsInHeadersCheck.cpp
+  ForwardDeclarationNamespaceCheck.cpp
   InaccurateEraseCheck.cpp
   InefficientAlgorithmCheck.cpp
   MacroParenthesesCheck.cpp
   MacroRepeatedSideEffectsCheck.cpp
   MiscTidyModule.cpp
-  MoveConstantArgumentCheck.cpp 
+  MoveConstantArgumentCheck.cpp
   MoveConstructorInitCheck.cpp
   NewDeleteOverloadsCheck.cpp
   NoexceptMoveConstructorCheck.cpp
Index: clang-tidy/misc/ForwardDeclarationNamespaceCheck.h
===================================================================
--- /dev/null
+++ clang-tidy/misc/ForwardDeclarationNamespaceCheck.h
@@ -0,0 +1,58 @@
+//===--- ForwardDeclarationNamespaceCheck.h - clang-tidy --------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_FORWARDDECLARATIONNAMESPACECHECK_H_
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_FORWARDDECLARATIONNAMESPACECHECK_H_
+
+#include <map>
+#include <set>
+#include <string>
+#include <vector>
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+/// Checks if an unused forward declaration is in a wrong namespace
+///
+/// The check inspects all unused forward declarations and checks if there is
+/// any declaration/definition with the same name existing, which could indicate
+/// that the forward declaration is in a potentially wrong namespace.
+///
+/// \code
+///   namespace na { struct A; }
+///   namespace nb { struct A {} };
+///   nb::A a;
+///   // warning : no definition found for 'A', but a definition with the same
+///   name 'A' found in another namespace 'nb::'
+/// \endcode
+///
+/// This check can only generate warning but can't suggest fix at this point.
+
+class ForwardDeclarationNamespaceCheck : public ClangTidyCheck {
+public:
+ ForwardDeclarationNamespaceCheck(StringRef Name, ClangTidyContext *Context)
+     : ClangTidyCheck(Name, Context) {}
+ void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+ void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+ void onEndOfTranslationUnit() override;
+private:
+ std::map<std::string, std::vector<const CXXRecordDecl *>>
+     DeclNameToDefinitions;
+ std::map<std::string, std::vector<const CXXRecordDecl *>>
+     DeclNameToDeclarations;
+ std::set<const Type *> FriendTypes;
+};
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_FORWARDDECLARATIONNAMESPACECHECK_H_
Index: clang-tidy/misc/ForwardDeclarationNamespaceCheck.cpp
===================================================================
--- /dev/null
+++ clang-tidy/misc/ForwardDeclarationNamespaceCheck.cpp
@@ -0,0 +1,180 @@
+//===--- ForwardDeclarationNamespaceCheck.cpp - clang-tidy
+//--------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "ForwardDeclarationNamespaceCheck.h"
+#include <stack>
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/Decl.h"
+#include "clang/AST/DeclBase.h"
+#include "clang/AST/DeclCXX.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+void ForwardDeclarationNamespaceCheck::registerMatchers(MatchFinder *Finder) {
+  // Matches all class declarations/definitions *EXCEPT*
+  //  1. implicit class. Eg. class A {}; has implicit class A declared inside A
+  //  2. classes declared/defined inside classes(can be detected by compiler)
+  //  3. template class declaration, template instantiation or
+  //  specialization(NOTE: extern specialization is filtered out by
+  //  "unless(hasAncestor(cxxRecordDecl()))"
+  auto IsInSpecialization = hasAncestor(
+      decl(anyOf(cxxRecordDecl(isExplicitTemplateSpecialization()),
+                 functionDecl(isExplicitTemplateSpecialization()))));
+  Finder->addMatcher(
+      cxxRecordDecl(
+          hasParent(decl(anyOf(namespaceDecl(), translationUnitDecl()))),
+          unless(isImplicit()), unless(hasAncestor(cxxRecordDecl())),
+          unless(isInstantiated()), unless(IsInSpecialization),
+          unless(classTemplateSpecializationDecl()))
+          .bind("record_decl"),
+      this);
+  // Match all friend declaration. Classes used in friend declarations are not
+  // marked as referenced in AST. We need to record all record classes used in
+  // friend declarations
+  Finder->addMatcher(friendDecl().bind("friend_decl"), this);
+}
+
+void ForwardDeclarationNamespaceCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  if (auto *RecordDecl = Result.Nodes.getNodeAs<CXXRecordDecl>("record_decl")) {
+    std::string DeclName(RecordDecl->getName().data());
+    if (RecordDecl->isThisDeclarationADefinition()) {
+      DeclNameToDefinitions[DeclName].push_back(RecordDecl);
+    } else {
+      // if a declaration has no definition, the definition could be in another
+      // namespace(a wrong namespace)
+      // NOTE: even a declaration does have definition, we still need it to
+      // compare with other declarations
+      DeclNameToDeclarations[DeclName].push_back(RecordDecl);
+    }
+  } else {
+    auto *decl = Result.Nodes.getNodeAs<FriendDecl>("friend_decl");
+    assert(decl && "Decl is neither record_decl nor friend decl!");
+    // classes used in friend delarations are not marked referenced in AST
+    // so we need to check classes used in friend declarations manually to
+    // reduce false positive
+    // For example, in
+    //      struct A;
+    //      struct B { friend A; };
+    // A will not be marked as "referenced" in the AST
+    if (const TypeSourceInfo *tsi = decl->getFriendType()) {
+      QualType desugared = tsi->getType().getDesugaredType(*Result.Context);
+      FriendTypes.insert(desugared.getTypePtr());
+    }
+  }
+}
+
+namespace {
+
+bool haveSameNamespaceOrTranslationUnit(const CXXRecordDecl *Decl1,
+                                        const CXXRecordDecl *Decl2) {
+  const DeclContext *ParentDecl1 = Decl1->getLexicalParent();
+  const DeclContext *ParentDecl2 = Decl2->getLexicalParent();
+  // Since we only matched decls whose parent is Namespace or TranslationUnit
+  // declaration, parent should be either a translation unit or namespace
+  if (ParentDecl1->getDeclKind() == Decl::TranslationUnit ||
+      ParentDecl2->getDeclKind() == Decl::TranslationUnit) {
+    return ParentDecl1 == ParentDecl2;
+  }
+  assert(ParentDecl1->getDeclKind() == Decl::Namespace &&
+         "ParentDecl1 declaration must be a namespace");
+  assert(ParentDecl2->getDeclKind() == Decl::Namespace &&
+         "ParentDecl2 declaration must be a namespace");
+  auto *Ns1 = NamespaceDecl::castFromDeclContext(ParentDecl1);
+  auto *Ns2 = NamespaceDecl::castFromDeclContext(ParentDecl2);
+  return Ns1->getOriginalNamespace() == Ns2->getOriginalNamespace();
+}
+
+std::string getNameOfNamespace(const CXXRecordDecl *Decl) {
+  std::stack<const NamespaceDecl *> Stk;
+  for (auto *ParentDecl = Decl->getLexicalParent();
+       ParentDecl->getDeclKind() != Decl::TranslationUnit;
+       ParentDecl = ParentDecl->getLexicalParent()) {
+    auto *NsDecl = dyn_cast<NamespaceDecl>(ParentDecl);
+    assert(NsDecl && "Cast to NamespaceDecl failed");
+    Stk.push(NsDecl);
+  }
+  std::string Ns = "";
+  while (!Stk.empty()) {
+    auto *namespaceDecl = Stk.top();
+    Stk.pop();
+    Ns += namespaceDecl->getName().data();
+    Ns += "::";
+  }
+  if (Ns.length() == 0) {
+    Ns = "Top level";
+  }
+  return Ns;
+}
+
+} // anonymous namespace
+
+void ForwardDeclarationNamespaceCheck::onEndOfTranslationUnit() {
+  // Iterate each group of declarations grouped by names.
+  for (auto const &Iterator : DeclNameToDeclarations) {
+    auto &Declarations = Iterator.second;
+    // if more than 1 declarations exist, check if all have the same namesapce
+    for (const auto *CurDecl : Declarations) {
+      if (CurDecl->hasDefinition() || CurDecl->isReferenced()) {
+        continue; // skip forward declarations that are used/referenced
+      }
+      if (FriendTypes.find(CurDecl->getTypeForDecl()) != FriendTypes.end()) {
+        continue; // skip forward declarations referenced as friend
+      }
+      if (CurDecl->getLocation().isMacroID() ||
+          CurDecl->getLocation().isInvalid()) {
+        continue;
+      }
+      // compare with all other declarations with the same name
+      for (auto *Decl : Declarations) {
+        if (Decl == CurDecl) {
+          continue; // don't compare with self
+        }
+        if (!CurDecl->hasDefinition() &&
+            !haveSameNamespaceOrTranslationUnit(CurDecl, Decl)) {
+          diag(CurDecl->getLocation(),
+               "declaration is never referenced, but a declaration with the "
+               "same name '%0' found in another namespace '%1'")
+              << CurDecl->getName() << getNameOfNamespace(Decl);
+          diag(Decl->getLocation(), "a declaration of '%0' is found here",
+               DiagnosticIDs::Note)
+              << Decl->getName();
+          break; // FIXME: if want location of all other decls, don't break
+        }
+      }
+      // check if a definition in another namespace exists
+      auto DeclName = CurDecl->getNameAsString();
+      if (DeclNameToDefinitions.find(DeclName) == DeclNameToDefinitions.end()) {
+        continue; // No definition in this translation unit, skip it
+      }
+      // make a warning for each definition with the same name but different ns
+      auto &definitions = DeclNameToDefinitions[DeclName];
+      for (const auto *Def : definitions) {
+        diag(CurDecl->getLocation(),
+             "no definition found for '%0', but a definition with "
+             "the same name '%1' found in another namespace '%2'")
+            << CurDecl->getName() << Def->getName() << getNameOfNamespace(Def);
+        diag(Def->getLocation(), "a definition of '%0' is found here",
+             DiagnosticIDs::Note)
+            << Def->getName();
+      }
+    }
+  }
+}
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang
Index: clang-tidy/misc/MiscTidyModule.cpp
===================================================================
--- clang-tidy/misc/MiscTidyModule.cpp
+++ clang-tidy/misc/MiscTidyModule.cpp
@@ -15,6 +15,7 @@
 #include "AssignOperatorSignatureCheck.h"
 #include "BoolPointerImplicitConversionCheck.h"
 #include "DefinitionsInHeadersCheck.h"
+#include "ForwardDeclarationNamespaceCheck.h"
 #include "InaccurateEraseCheck.h"
 #include "InefficientAlgorithmCheck.h"
 #include "MacroParenthesesCheck.h"
@@ -52,6 +53,8 @@
         "misc-bool-pointer-implicit-conversion");
     CheckFactories.registerCheck<DefinitionsInHeadersCheck>(
         "misc-definitions-in-headers");
+    CheckFactories.registerCheck<ForwardDeclarationNamespaceCheck>(
+        "misc-forward-declaration-namespace");
     CheckFactories.registerCheck<InaccurateEraseCheck>(
         "misc-inaccurate-erase");
     CheckFactories.registerCheck<InefficientAlgorithmCheck>(
Index: docs/clang-tidy/checks/misc-forward-declaration-namespace.rst
===================================================================
--- /dev/null
+++ docs/clang-tidy/checks/misc-forward-declaration-namespace.rst
@@ -0,0 +1,20 @@
+.. title:: clang-tidy - misc-forward-declaration-namespace
+
+misc-forward-declaration-namespace
+==================================
+
+
+Checks if an unused forward declaration is in a wrong namespace.
+
+The check inspects all unused forward declarations and checks if there is any
+declaration/definition with the same name existing, which could indicate that
+the forward declaration is in a potentially wrong namespace.
+
+.. code:: c++
+  namespace na { struct A; }
+  namespace nb { struct A {}; }
+  nb::A a;
+  // warning : no definition found for 'A', but a definition with the same name
+  // 'A' found in another namespace 'nb::'
+
+This check can only generate warning but can't suggest fix at this point.
Index: test/clang-tidy/misc-forward-declaration-namespace.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/misc-forward-declaration-namespace.cpp
@@ -0,0 +1,154 @@
+// RUN: %check_clang_tidy %s misc-forward-declaration-namespace %t 
+
+namespace {
+// Delaration in the wrong namespace
+class T_A;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration is never referenced, but a declaration with the same name 'T_A' found in another namespace 'na::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a declaration of 'T_A' is found here
+// CHECK-MESSAGES: :[[@LINE-3]]:7: warning: no definition found for 'T_A', but a definition with the same name 'T_A' found in another namespace 'Top level' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a definition of 'T_A' is found here
+}
+
+namespace na {
+// Delaration in the wrong namespace
+class T_A;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration is never referenced, but a declaration with the same name 'T_A' found in another namespace '::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a declaration of 'T_A' is found here
+// CHECK-MESSAGES: :[[@LINE-3]]:7: warning: no definition found for 'T_A', but a definition with the same name 'T_A' found in another namespace 'Top level' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a definition of 'T_A' is found here
+}
+
+class T_A;
+
+class T_A {
+  int x;
+};
+
+
+namespace na {
+class T_B;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration is never referenced, but a declaration with the same name 'T_B' found in another namespace 'nb::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a declaration of 'T_B' is found here
+// CHECK-MESSAGES: :[[@LINE-3]]:7: warning: no definition found for 'T_B', but a definition with the same name 'T_B' found in another namespace 'nb::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a definition of 'T_B' is found here
+}
+
+namespace nb {
+class T_B;
+}
+
+namespace nb {
+class T_B {
+  int x;
+};
+}
+
+namespace na {
+class T_B;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration is never referenced, but a declaration with the same name 'T_B' found in another namespace 'nb::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a declaration of 'T_B' is found here
+// CHECK-MESSAGES: :[[@LINE-3]]:7: warning: no definition found for 'T_B', but a definition with the same name 'T_B' found in another namespace 'nb::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a definition of 'T_B' is found here
+}
+
+// simple forward declaration. although never used, but no same declaration
+// found in other namespace
+class OUTSIDER;
+
+namespace na {
+// Referenced declaration, no warning
+class OUTSIDER_1;
+}
+
+void f(na::OUTSIDER_1);
+
+namespace nc {
+// referenced as friend
+class OUTSIDER_1;
+
+class OOP {
+  friend struct OUTSIDER_1;
+};
+}
+
+namespace nd {
+class OUTSIDER_1;
+void f(OUTSIDER_1 *);
+}
+
+namespace nb {
+class OUTSIDER_1;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration is never referenced, but a declaration with the same name 'OUTSIDER_1' found in another namespace 'na::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a declaration of 'OUTSIDER_1' is found here
+}
+
+
+namespace na {
+template<typename T>
+class T_C;
+}
+
+namespace nb {
+// FIXME: this is an error, but we don't consider template class declaration now
+template<typename T>
+class T_C;
+}
+
+namespace na {
+template<typename T>
+class T_C {
+  int x;
+};
+}
+
+namespace na {
+
+template<typename T>
+  class T_TEMP {
+    template<typename _Tp1>
+      struct rebind 
+      { typedef T_TEMP<_Tp1> other; };
+};
+
+// Ignoring class template specialization
+template class T_TEMP<char>;
+} 
+
+namespace nb {
+
+template<typename T> 
+  class T_TEMP_1 {
+    template<typename _Tp1>
+      struct rebind 
+      { typedef T_TEMP_1<_Tp1> other; };
+};
+
+// Ignoring class template specialization
+extern template class T_TEMP_1<char>;
+}
+
+namespace nd {
+class D;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: declaration is never referenced, but a declaration with the same name 'D' found in another namespace 'nd::ne::' [misc-forward-declaration-namespace]
+// CHECK-MESSAGES: note: a declaration of 'D' is found here
+}
+
+namespace nd {
+namespace ne {
+class D;
+}
+}
+
+int f(nd::ne::D &d);
+
+
+// Check if we can ignore this case
+template <typename... Args>
+class Observer {
+    class Impl;
+};
+
+template <typename... Args>
+class Observer<Args...>::Impl {
+
+};