Index: include/clang/AST/ASTImporter.h
===================================================================
--- include/clang/AST/ASTImporter.h
+++ include/clang/AST/ASTImporter.h
@@ -43,6 +43,15 @@
 class TypeSourceInfo;
 class Attr;
 
+  // \brief Returns with a list of declarations started from the canonical decl
+  // then followed by subsequent decls in the translation unit.
+  // This gives a canonical list for each entry in the redecl chain.
+  // `Decl::redecls()` gives a list of decls which always start from the
+  // previous decl and the next item is actually the previous item in the order
+  // of source locations.  Thus, `Decl::redecls()` gives different lists for
+  // the different entries in a given redecl chain.
+  llvm::SmallVector<Decl*, 2> getCanonicalForwardRedeclChain(Decl* D);
+
   /// Imports selected nodes from one AST context into another context,
   /// merging AST nodes where appropriate.
   class ASTImporter {
Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -71,6 +71,25 @@
 
 namespace clang {
 
+  template <class T>
+  SmallVector<Decl*, 2>
+  getCanonicalForwardRedeclChain(Redeclarable<T>* D) {
+    SmallVector<Decl*, 2> Redecls;
+    for (auto *R : D->getFirstDecl()->redecls()) {
+      if (R != D->getFirstDecl())
+        Redecls.push_back(R);
+    }
+    Redecls.push_back(D->getFirstDecl());
+    std::reverse(Redecls.begin(), Redecls.end());
+    return Redecls;
+  }
+
+  SmallVector<Decl*, 2> getCanonicalForwardRedeclChain(Decl* D) {
+    // Currently only FunctionDecl is supported
+    auto FD = cast<FunctionDecl>(D);
+    return getCanonicalForwardRedeclChain<FunctionDecl>(FD);
+  }
+
   class ASTNodeImporter : public TypeVisitor<ASTNodeImporter, QualType>,
                           public DeclVisitor<ASTNodeImporter, Decl *>,
                           public StmtVisitor<ASTNodeImporter, Stmt *> {
@@ -195,6 +214,12 @@
                                         const InContainerTy &Container,
                                         TemplateArgumentListInfo &Result);
 
+    using TemplateArgsTy = SmallVector<TemplateArgument, 8>;
+    using OptionalTemplateArgsTy = Optional<TemplateArgsTy>;
+    std::tuple<FunctionTemplateDecl *, OptionalTemplateArgsTy>
+    ImportFunctionTemplateWithTemplateArgsFromSpecialization(
+        FunctionDecl *FromFD);
+
     bool ImportTemplateInformation(FunctionDecl *FromFD, FunctionDecl *ToFD);
 
     bool IsStructuralMatch(RecordDecl *FromRecord, RecordDecl *ToRecord,
@@ -408,6 +433,8 @@
 
     // Importing overrides.
     void ImportOverrides(CXXMethodDecl *ToMethod, CXXMethodDecl *FromMethod);
+
+    FunctionDecl *FindFunctionTemplateSpecialization(FunctionDecl *FromFD);
   };
 
 template <typename InContainerTy>
@@ -437,6 +464,25 @@
                                         From.arguments(), Result);
 }
 
+std::tuple<FunctionTemplateDecl *, ASTNodeImporter::OptionalTemplateArgsTy>
+ASTNodeImporter::ImportFunctionTemplateWithTemplateArgsFromSpecialization(
+    FunctionDecl *FromFD) {
+  assert(FromFD->getTemplatedKind() ==
+         FunctionDecl::TK_FunctionTemplateSpecialization);
+  auto *FTSInfo = FromFD->getTemplateSpecializationInfo();
+  auto *Template = cast_or_null<FunctionTemplateDecl>(
+      Importer.Import(FTSInfo->getTemplate()));
+
+  // Import template arguments.
+  auto TemplArgs = FTSInfo->TemplateArguments->asArray();
+  TemplateArgsTy ToTemplArgs;
+  if (ImportTemplateArguments(TemplArgs.data(), TemplArgs.size(),
+                              ToTemplArgs)) // Error during import.
+    return std::make_tuple(Template, OptionalTemplateArgsTy());
+
+  return std::make_tuple(Template, ToTemplArgs);
+}
+
 } // namespace clang
 
 //----------------------------------------------------------------------------
@@ -2252,23 +2298,17 @@
   }
 
   case FunctionDecl::TK_FunctionTemplateSpecialization: {
-    auto *FTSInfo = FromFD->getTemplateSpecializationInfo();
-    auto *Template = cast_or_null<FunctionTemplateDecl>(
-        Importer.Import(FTSInfo->getTemplate()));
-    if (!Template)
-      return true;
-    TemplateSpecializationKind TSK = FTSInfo->getTemplateSpecializationKind();
-
-    // Import template arguments.
-    auto TemplArgs = FTSInfo->TemplateArguments->asArray();
-    SmallVector<TemplateArgument, 8> ToTemplArgs;
-    if (ImportTemplateArguments(TemplArgs.data(), TemplArgs.size(),
-                                ToTemplArgs))
+    FunctionTemplateDecl* Template;
+    OptionalTemplateArgsTy ToTemplArgs;
+    std::tie(Template, ToTemplArgs) =
+        ImportFunctionTemplateWithTemplateArgsFromSpecialization(FromFD);
+    if (!Template || !ToTemplArgs)
       return true;
 
     TemplateArgumentList *ToTAList = TemplateArgumentList::CreateCopy(
-          Importer.getToContext(), ToTemplArgs);
+          Importer.getToContext(), *ToTemplArgs);
 
+    auto *FTSInfo = FromFD->getTemplateSpecializationInfo();
     TemplateArgumentListInfo ToTAInfo;
     const auto *FromTAArgsAsWritten = FTSInfo->TemplateArgumentsAsWritten;
     if (FromTAArgsAsWritten)
@@ -2277,6 +2317,7 @@
 
     SourceLocation POI = Importer.Import(FTSInfo->getPointOfInstantiation());
 
+    TemplateSpecializationKind TSK = FTSInfo->getTemplateSpecializationKind();
     ToFD->setFunctionTemplateSpecialization(
         Template, ToTAList, /* InsertPos= */ nullptr,
         TSK, FromTAArgsAsWritten ? &ToTAInfo : nullptr, POI);
@@ -2312,7 +2353,31 @@
   llvm_unreachable("All cases should be covered!");
 }
 
+FunctionDecl *
+ASTNodeImporter::FindFunctionTemplateSpecialization(FunctionDecl *FromFD) {
+  FunctionTemplateDecl* Template;
+  OptionalTemplateArgsTy ToTemplArgs;
+  std::tie(Template, ToTemplArgs) =
+      ImportFunctionTemplateWithTemplateArgsFromSpecialization(FromFD);
+  if (!Template || !ToTemplArgs)
+    return nullptr;
+
+  void *InsertPos = nullptr;
+  auto *FoundSpec = Template->findSpecialization(*ToTemplArgs, InsertPos);
+  return FoundSpec;
+}
+
 Decl *ASTNodeImporter::VisitFunctionDecl(FunctionDecl *D) {
+
+  SmallVector<Decl*, 2> Redecls = getCanonicalForwardRedeclChain(D);
+  auto RedeclIt = Redecls.begin();
+  // Import the first part of the decl chain. I.e. import all previous
+  // declarations starting from the canonical decl.
+  for (; RedeclIt != Redecls.end() && *RedeclIt != D; ++RedeclIt)
+    if (!Importer.Import(*RedeclIt))
+      return nullptr;
+  assert(*RedeclIt == D);
+
   // Import the major distinguishing characteristics of this function.
   DeclContext *DC, *LexicalDC;
   DeclarationName Name;
@@ -2323,13 +2388,27 @@
   if (ToD)
     return ToD;
 
-  const FunctionDecl *FoundWithoutBody = nullptr;
-
+  const FunctionDecl *FoundByLookup = nullptr;
+
+  // If this is a function template specialization, then try to find the same
+  // existing specialization in the "to" context. The localUncachedLookup
+  // below will not find any specialization, but would find the primary
+  // template; thus, we have to skip normal lookup in case of specializations.
+  // FIXME handle member function templates (TK_MemberSpecialization) similarly?
+  if (D->getTemplatedKind() ==
+      FunctionDecl::TK_FunctionTemplateSpecialization) {
+    if (FunctionDecl *FoundFunction = FindFunctionTemplateSpecialization(D)) {
+      if (D->doesThisDeclarationHaveABody() &&
+          FoundFunction->hasBody())
+        return Importer.Imported(D, FoundFunction);
+      FoundByLookup = FoundFunction;
+    }
+  }
   // Try to find a function in our own ("to") context with the same name, same
   // type, and in the same context as the function we're importing.
-  if (!LexicalDC->isFunctionOrMethod()) {
+  else if (!LexicalDC->isFunctionOrMethod()) {
     SmallVector<NamedDecl *, 4> ConflictingDecls;
-    unsigned IDNS = Decl::IDNS_Ordinary;
+    unsigned IDNS = Decl::IDNS_Ordinary | Decl::IDNS_OrdinaryFriend;
     SmallVector<NamedDecl *, 2> FoundDecls;
     DC->getRedeclContext()->localUncachedLookup(Name, FoundDecls);
     for (auto *FoundDecl : FoundDecls) {
@@ -2341,15 +2420,11 @@
             D->hasExternalFormalLinkage()) {
           if (Importer.IsStructurallyEquivalent(D->getType(), 
                                                 FoundFunction->getType())) {
-            // FIXME: Actually try to merge the body and other attributes.
-            const FunctionDecl *FromBodyDecl = nullptr;
-            D->hasBody(FromBodyDecl);
-            if (D == FromBodyDecl && !FoundFunction->hasBody()) {
-              // This function is needed to merge completely.
-              FoundWithoutBody = FoundFunction;
+              if (D->doesThisDeclarationHaveABody() &&
+                  FoundFunction->hasBody())
+                return Importer.Imported(D, FoundFunction);
+              FoundByLookup = FoundFunction;
               break;
-            }
-            return Importer.Imported(D, FoundFunction);
           }
 
           // FIXME: Check for overloading more carefully, e.g., by boosting
@@ -2499,9 +2574,9 @@
   }
   ToFunction->setParams(Parameters);
 
-  if (FoundWithoutBody) {
+  if (FoundByLookup) {
     auto *Recent = const_cast<FunctionDecl *>(
-          FoundWithoutBody->getMostRecentDecl());
+          FoundByLookup->getMostRecentDecl());
     ToFunction->setPreviousDecl(Recent);
   }
 
@@ -2523,10 +2598,11 @@
     ToFunction->setType(T);
   }
 
-  // Import the body, if any.
-  if (Stmt *FromBody = D->getBody()) {
-    if (Stmt *ToBody = Importer.Import(FromBody)) {
-      ToFunction->setBody(ToBody);
+  if (D->doesThisDeclarationHaveABody()) {
+    if (Stmt *FromBody = D->getBody()) {
+      if (Stmt *ToBody = Importer.Import(FromBody)) {
+        ToFunction->setBody(ToBody);
+      }
     }
   }
 
@@ -2536,14 +2612,29 @@
   if (ImportTemplateInformation(D, ToFunction))
     return nullptr;
 
-  // Add this function to the lexical context.
-  // NOTE: If the function is templated declaration, it should be not added into
-  // LexicalDC. But described template is imported during import of
-  // FunctionTemplateDecl (it happens later). So, we use source declaration
-  // to determine if we should add the result function.
-  if (!D->getDescribedFunctionTemplate())
+  bool IsFriend = D->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend);
+
+  // TODO Can we generalize this approach to other AST nodes as well?
+  if (D->getDeclContext()->containsDecl(D))
+    DC->addDeclInternal(ToFunction);
+  if (DC != LexicalDC && D->getLexicalDeclContext()->containsDecl(D))
     LexicalDC->addDeclInternal(ToFunction);
 
+  // Friend declaration's lexical context is the befriending class, but the
+  // semantic context is the enclosing scope of the befriending class.
+  // We want the friend functions to be found in the semantic context by lookup.
+  // FIXME should we handle this generically in VisitFriendDecl?
+  // In Other cases when LexicalDC != DC we don't want it to be added,
+  // e.g out-of-class definitions like void B::f() {} .
+  if (LexicalDC != DC && IsFriend) {
+    DC->makeDeclVisibleInContext(ToFunction);
+  }
+
+  // Import the rest of the chain. I.e. import all subsequent declarations.
+  for (++RedeclIt; RedeclIt != Redecls.end(); ++RedeclIt)
+    if (!Importer.Import(*RedeclIt))
+      return nullptr;
+
   if (auto *FromCXXMethod = dyn_cast<CXXMethodDecl>(D))
     ImportOverrides(cast<CXXMethodDecl>(ToFunction), FromCXXMethod);
 
Index: lib/AST/DeclBase.cpp
===================================================================
--- lib/AST/DeclBase.cpp
+++ lib/AST/DeclBase.cpp
@@ -1343,6 +1343,8 @@
 }
 
 bool DeclContext::containsDecl(Decl *D) const {
+  if (hasExternalLexicalStorage())
+    LoadLexicalDeclsFromExternalStorage();
   return (D->getLexicalDeclContext() == this &&
           (D->NextInContextAndBits.getPointer() || D == LastDecl));
 }
Index: test/ASTMerge/class/test.cpp
===================================================================
--- test/ASTMerge/class/test.cpp
+++ test/ASTMerge/class/test.cpp
@@ -13,12 +13,12 @@
 // CHECK: class1.cpp:19:3: note: enumerator 'b' with value 1 here
 // CHECK: class2.cpp:12:3: note: enumerator 'a' with value 0 here
 
-// CHECK: class1.cpp:36:8: warning: type 'F2' has incompatible definitions in different translation units
-// CHECK: class1.cpp:39:3: note: friend declared here
-// CHECK: class2.cpp:30:8: note: no corresponding friend here
-
 // CHECK: class1.cpp:43:8: warning: type 'F3' has incompatible definitions in different translation units
 // CHECK: class1.cpp:46:3: note: friend declared here
 // CHECK: class2.cpp:36:8: note: no corresponding friend here
 
+// CHECK: class1.cpp:36:8: warning: type 'F2' has incompatible definitions in different translation units
+// CHECK: class1.cpp:39:3: note: friend declared here
+// CHECK: class2.cpp:30:8: note: no corresponding friend here
+
 // CHECK: 4 warnings generated.
Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -20,7 +20,7 @@
 
 #include "DeclMatcher.h"
 #include "Language.h"
-#include "gtest/gtest.h"
+#include "gmock/gmock.h"
 #include "llvm/ADT/StringMap.h"
 
 namespace clang {
@@ -428,6 +428,48 @@
 struct ImportType : TestImportBase {};
 struct ImportDecl : TestImportBase {};
 
+struct CanonicalRedeclChain : ASTImporterTestBase {};
+
+TEST_P(CanonicalRedeclChain, ShouldBeConsequentWithMatchers) {
+  Decl *FromTU = getTuDecl("void f();", Lang_CXX);
+  auto Pattern = functionDecl(hasName("f"));
+  auto *D0 = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+  auto Redecls = getCanonicalForwardRedeclChain(D0);
+  ASSERT_EQ(Redecls.size(), 1u);
+  EXPECT_EQ(D0, Redecls[0]);
+}
+
+TEST_P(CanonicalRedeclChain, ShouldBeConsequentWithMatchers2) {
+  Decl *FromTU = getTuDecl("void f(); void f(); void f();", Lang_CXX);
+  auto Pattern = functionDecl(hasName("f"));
+  auto *D0 = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+  auto *D2 = LastDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+  FunctionDecl *D1 = D2->getPreviousDecl();
+
+  auto Redecls = getCanonicalForwardRedeclChain(D0);
+  ASSERT_EQ(Redecls.size(), 3u);
+  EXPECT_EQ(D0, Redecls[0]);
+  EXPECT_EQ(D1, Redecls[1]);
+  EXPECT_EQ(D2, Redecls[2]);
+}
+
+TEST_P(CanonicalRedeclChain, ShouldBeSameForAllDeclInTheChain) {
+  Decl *FromTU = getTuDecl("void f(); void f(); void f();", Lang_CXX);
+  auto Pattern = functionDecl(hasName("f"));
+  auto *D0 = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+  auto *D2 = LastDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+  FunctionDecl *D1 = D2->getPreviousDecl();
+
+  auto RedeclsD0 = getCanonicalForwardRedeclChain(D0);
+  auto RedeclsD1 = getCanonicalForwardRedeclChain(D1);
+  auto RedeclsD2 = getCanonicalForwardRedeclChain(D2);
+
+  EXPECT_THAT(RedeclsD0, ::testing::ContainerEq(RedeclsD1));
+  EXPECT_THAT(RedeclsD1, ::testing::ContainerEq(RedeclsD2));
+}
+
+
 TEST_P(ImportExpr, ImportStringLiteral) {
   MatchVerifier<Decl> Verifier;
   testImport("void declToImport() { \"foo\"; }",
@@ -1673,34 +1715,6 @@
 struct ImportFunctions : ASTImporterTestBase {};
 
 TEST_P(ImportFunctions,
-       PrototypeShouldBeImportedAsAPrototypeWhenThereIsNoDefinition) {
-  Decl *FromTU = getTuDecl("void f();", Lang_CXX);
-  auto Pattern = functionDecl(hasName("f"));
-  FunctionDecl *FromD =
-      FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
-
-  Decl *ImportedD = Import(FromD, Lang_CXX);
-  Decl *ToTU = ImportedD->getTranslationUnitDecl();
-
-  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
-  EXPECT_TRUE(!cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
-}
-
-TEST_P(ImportFunctions,
-       PrototypeShouldBeImportedAsDefintionWhenThereIsADefinition) {
-  Decl *FromTU = getTuDecl("void f(); void f() {}", Lang_CXX);
-  auto Pattern = functionDecl(hasName("f"));
-  FunctionDecl *FromD = // Prototype
-      FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
-
-  Decl *ImportedD = Import(FromD, Lang_CXX);
-  Decl *ToTU = ImportedD->getTranslationUnitDecl();
-
-  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
-  EXPECT_TRUE(cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
-}
-
-TEST_P(ImportFunctions,
        DefinitionShouldBeImportedAsDefintionWhenThereIsAPrototype) {
   Decl *FromTU = getTuDecl("void f(); void f() {}", Lang_CXX);
   auto Pattern = functionDecl(hasName("f"));
@@ -1710,7 +1724,7 @@
   Decl *ImportedD = Import(FromD, Lang_CXX);
   Decl *ToTU = ImportedD->getTranslationUnitDecl();
 
-  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
   EXPECT_TRUE(cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
 }
 
@@ -1727,30 +1741,40 @@
   EXPECT_TRUE(cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
 }
 
-TEST_P(ImportFunctions, DISABLED_ImportPrototypeOfRecursiveFunction) {
+TEST_P(ImportFunctions, ImportPrototypeOfRecursiveFunction) {
   Decl *FromTU = getTuDecl("void f(); void f() { f(); }", Lang_CXX);
   auto Pattern = functionDecl(hasName("f"));
-  FunctionDecl *PrototypeFD =
-      FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+  auto *From =
+      FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern); // Proto
 
-  Decl *ImportedD = Import(PrototypeFD, Lang_CXX);
+  Decl *ImportedD = Import(From, Lang_CXX);
   Decl *ToTU = ImportedD->getTranslationUnitDecl();
 
-  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
-  EXPECT_TRUE(cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_FALSE(To0->doesThisDeclarationHaveABody());
+  EXPECT_TRUE(To1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(To1->getPreviousDecl(), To0);
 }
 
 TEST_P(ImportFunctions, ImportDefinitionOfRecursiveFunction) {
   Decl *FromTU = getTuDecl("void f(); void f() { f(); }", Lang_CXX);
   auto Pattern = functionDecl(hasName("f"));
-  FunctionDecl *DefinitionFD =
-      LastDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+  auto *From =
+      LastDeclMatcher<FunctionDecl>().match(FromTU, Pattern); // Def
 
-  Decl *ImportedD = Import(DefinitionFD, Lang_CXX);
+  Decl *ImportedD = Import(From, Lang_CXX);
   Decl *ToTU = ImportedD->getTranslationUnitDecl();
 
-  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
-  EXPECT_TRUE(cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To1);
+  EXPECT_FALSE(To0->doesThisDeclarationHaveABody());
+  EXPECT_TRUE(To1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(To1->getPreviousDecl(), To0);
 }
 
 TEST_P(ImportFunctions, ImportPrototypes) {
@@ -1759,23 +1783,48 @@
   Decl *ImportedD;
   {
     Decl *FromTU = getTuDecl("void f();", Lang_CXX, "input0.cc");
-    FunctionDecl *FromD =
-        FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
 
     ImportedD = Import(FromD, Lang_CXX);
   }
-  Decl *ImportedD1;
   {
     Decl *FromTU = getTuDecl("void f();", Lang_CXX, "input1.cc");
-    FunctionDecl *FromD =
-        FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
-    ImportedD1 = Import(FromD, Lang_CXX);
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    Import(FromD, Lang_CXX);
   }
 
-  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  Decl *ToTU = ImportedD->getTranslationUnitDecl();
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_FALSE(To0->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(To1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(To1->getPreviousDecl(), To0);
+}
+
+TEST_P(ImportFunctions, ImportDefinitions) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  Decl *ImportedD;
+  {
+    Decl *FromTU = getTuDecl("void f(){}", Lang_CXX, "input0.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    ImportedD = Import(FromD, Lang_CXX);
+  }
+  {
+    Decl *FromTU = getTuDecl("void f(){};", Lang_CXX, "input1.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    Import(FromD, Lang_CXX);
+  }
+
+  Decl *ToTU = ImportedD->getTranslationUnitDecl();
+
   EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
-  EXPECT_EQ(ImportedD, ImportedD1);
-  EXPECT_TRUE(!cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_TRUE(To0->doesThisDeclarationHaveABody());
 }
 
 TEST_P(ImportFunctions, ImportDefinitionThenPrototype) {
@@ -1784,23 +1833,24 @@
   Decl *ImportedD;
   {
     Decl *FromTU = getTuDecl("void f(){}", Lang_CXX, "input0.cc");
-    FunctionDecl *FromD =
-        FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
-
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
     ImportedD = Import(FromD, Lang_CXX);
   }
-  Decl *ImportedD1;
   {
     Decl *FromTU = getTuDecl("void f();", Lang_CXX, "input1.cc");
-    FunctionDecl *FromD =
-        FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
-    ImportedD1 = Import(FromD, Lang_CXX);
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    Import(FromD, Lang_CXX);
   }
 
   Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
-  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
-  EXPECT_EQ(ImportedD, ImportedD1);
-  EXPECT_TRUE(cast<FunctionDecl>(ImportedD)->doesThisDeclarationHaveABody());
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_TRUE(To0->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(To1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(To1->getPreviousDecl(), To0);
 }
 
 TEST_P(ImportFunctions, ImportPrototypeThenDefinition) {
@@ -1823,38 +1873,40 @@
   Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
   ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
   FunctionDecl *ProtoD = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
-  EXPECT_TRUE(!ProtoD->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(ProtoD->doesThisDeclarationHaveABody());
   FunctionDecl *DefinitionD =
       LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
   EXPECT_TRUE(DefinitionD->doesThisDeclarationHaveABody());
   EXPECT_EQ(DefinitionD->getPreviousDecl(), ProtoD);
 }
 
-TEST_P(ImportFunctions, DISABLED_ImportPrototypeThenProtoAndDefinition) {
+TEST_P(ImportFunctions, ImportPrototypeThenProtoAndDefinition) {
   auto Pattern = functionDecl(hasName("f"));
 
   {
     Decl *FromTU = getTuDecl("void f();", Lang_CXX, "input0.cc");
-    FunctionDecl *FromD =
-        FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
-
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
     Import(FromD, Lang_CXX);
   }
   {
     Decl *FromTU = getTuDecl("void f(); void f(){}", Lang_CXX, "input1.cc");
-    FunctionDecl *FromD =
-        FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
     Import(FromD, Lang_CXX);
   }
 
   Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
-  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 3u);
   FunctionDecl *ProtoD = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
-  EXPECT_TRUE(!ProtoD->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(ProtoD->doesThisDeclarationHaveABody());
+
   FunctionDecl *DefinitionD =
       LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
   EXPECT_TRUE(DefinitionD->doesThisDeclarationHaveABody());
-  EXPECT_EQ(DefinitionD->getPreviousDecl(), ProtoD);
+
+  EXPECT_TRUE(DefinitionD->getPreviousDecl());
+  EXPECT_FALSE(DefinitionD->getPreviousDecl()->doesThisDeclarationHaveABody());
+  EXPECT_EQ(DefinitionD->getPreviousDecl()->getPreviousDecl(), ProtoD);
 }
 
 TEST_P(ImportFunctions, OverriddenMethodsShouldBeImported) {
@@ -1894,6 +1946,202 @@
   EXPECT_TRUE(To->isVirtual());
 }
 
+TEST_P(ImportFunctions,
+       ImportDefinitionIfThereIsAnExistingDefinitionAndFwdDecl) {
+  Decl *ToTU = getToTuDecl(
+      R"(
+      void f() {}
+      void f();
+      )",
+      Lang_CXX);
+  ASSERT_EQ(1u,
+            DeclCounterWithPredicate<FunctionDecl>([](const FunctionDecl *FD) {
+              return FD->doesThisDeclarationHaveABody();
+            }).match(ToTU, functionDecl()));
+
+  Decl *FromTU = getTuDecl("void f() {}", Lang_CXX, "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, functionDecl());
+
+  Import(FromD, Lang_CXX);
+
+  EXPECT_EQ(1u,
+            DeclCounterWithPredicate<FunctionDecl>([](const FunctionDecl *FD) {
+              return FD->doesThisDeclarationHaveABody();
+            }).match(ToTU, functionDecl()));
+}
+
+struct ImportFriendFunctions : ImportFunctions {};
+
+TEST_P(ImportFriendFunctions, ImportFriendFunctionRedeclChainProto) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  Decl *FromTU = getTuDecl("struct X { friend void f(); };"
+                           "void f();",
+                           Lang_CXX,
+                           "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+  auto *ImportedD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  EXPECT_FALSE(ImportedD->doesThisDeclarationHaveABody());
+  auto *ToFD = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_FALSE(ToFD->doesThisDeclarationHaveABody());
+  EXPECT_EQ(ToFD->getPreviousDecl(), ImportedD);
+}
+
+TEST_P(ImportFriendFunctions,
+       ImportFriendFunctionRedeclChainProto_OutOfClassProtoFirst) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  Decl *FromTU = getTuDecl("void f();"
+                           "struct X { friend void f(); };",
+                           Lang_CXX, "input0.cc");
+  auto FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+  auto *ImportedD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  EXPECT_FALSE(ImportedD->doesThisDeclarationHaveABody());
+  auto *ToFD = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_FALSE(ToFD->doesThisDeclarationHaveABody());
+  EXPECT_EQ(ToFD->getPreviousDecl(), ImportedD);
+}
+
+TEST_P(ImportFriendFunctions, ImportFriendFunctionRedeclChainDef) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  Decl *FromTU = getTuDecl("struct X { friend void f(){} };"
+                           "void f();",
+                           Lang_CXX,
+                           "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+  auto *ImportedD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  EXPECT_TRUE(ImportedD->doesThisDeclarationHaveABody());
+  auto *ToFD = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_FALSE(ToFD->doesThisDeclarationHaveABody());
+  EXPECT_EQ(ToFD->getPreviousDecl(), ImportedD);
+}
+
+TEST_P(ImportFriendFunctions,
+       ImportFriendFunctionRedeclChainDef_OutOfClassDef) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  Decl *FromTU = getTuDecl("struct X { friend void f(); };"
+                           "void f(){}",
+                           Lang_CXX, "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+  auto *ImportedD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  EXPECT_FALSE(ImportedD->doesThisDeclarationHaveABody());
+  auto *ToFD = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ToFD->doesThisDeclarationHaveABody());
+  EXPECT_EQ(ToFD->getPreviousDecl(), ImportedD);
+}
+
+// This test is disabled, because ATM we create a redundant FunctionDecl.  We
+// start the import with the definition of `f` then we continue with the import
+// of the type of `f` which involves `X`. During the import of `X` we start
+// again the import of the definition of `f` and then finally we create the
+// node. But then in the first frame of `VisitFunctionDecl` we create a node
+// again since we do not check if such a node exists yet or not. This is being
+// fixed in a separate patch: https://reviews.llvm.org/D47632
+// FIXME enable this test once the above patch is approved.
+TEST_P(ImportFriendFunctions,
+    DISABLED_ImportFriendFunctionRedeclChainDefWithClass) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  Decl *FromTU = getTuDecl(
+      R"(
+        class X;
+        void f(X *x){}
+        class X{
+        friend void f(X *x);
+        };
+      )",
+      Lang_CXX, "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+  auto *ImportedD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  EXPECT_TRUE(ImportedD->doesThisDeclarationHaveABody());
+  auto *InClassFD = cast<FunctionDecl>(FirstDeclMatcher<FriendDecl>()
+                                              .match(ToTU, friendDecl())
+                                              ->getFriendDecl());
+  EXPECT_FALSE(InClassFD->doesThisDeclarationHaveABody());
+  EXPECT_EQ(InClassFD->getPreviousDecl(), ImportedD);
+  // The parameters must refer the same type
+  EXPECT_EQ((*InClassFD->param_begin())->getOriginalType(),
+            (*ImportedD->param_begin())->getOriginalType());
+}
+
+// This test is disabled, because ATM we create a redundant FunctionDecl.  We
+// start the import with the definition of `f` then we continue with the import
+// of the type of `f` which involves `X`. During the import of `X` we start
+// again the import of the definition of `f` and then finally we create the
+// node. But then in the first frame of `VisitFunctionDecl` we create a node
+// again since we do not check if such a node exists yet or not. This is being
+// fixed in a separate patch: https://reviews.llvm.org/D47632
+// FIXME enable this test once the above patch is approved.
+TEST_P(ImportFriendFunctions,
+       DISABLED_ImportFriendFunctionRedeclChainDefWithClass_ImportTheProto) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  Decl *FromTU = getTuDecl(
+      R"(
+        class X;
+        void f(X *x){}
+        class X{
+        friend void f(X *x);
+        };
+      )",
+      Lang_CXX, "input0.cc");
+  auto *FromD = LastDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+  auto *ImportedD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  EXPECT_FALSE(ImportedD->doesThisDeclarationHaveABody());
+  auto *OutOfClassFD = FirstDeclMatcher<FunctionDecl>().match(
+      ToTU, functionDecl(unless(hasParent(friendDecl()))));
+
+  EXPECT_TRUE(OutOfClassFD->doesThisDeclarationHaveABody());
+  EXPECT_EQ(ImportedD->getPreviousDecl(), OutOfClassFD);
+  // The parameters must refer the same type
+  EXPECT_EQ((*OutOfClassFD->param_begin())->getOriginalType(),
+            (*ImportedD->param_begin())->getOriginalType());
+}
+
+TEST_P(ImportFriendFunctions, ImportFriendFunctionFromMultipleTU) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  FunctionDecl *ImportedD;
+  {
+    Decl *FromTU =
+        getTuDecl("struct X { friend void f(){} };", Lang_CXX, "input0.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    ImportedD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  }
+  FunctionDecl *ImportedD1;
+  {
+    Decl *FromTU = getTuDecl("void f();", Lang_CXX, "input1.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    ImportedD1 = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  }
+
+  Decl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  EXPECT_TRUE(ImportedD->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(ImportedD1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(ImportedD1->getPreviousDecl(), ImportedD);
+}
+
 AST_MATCHER_P(TagDecl, hasTypedefForAnonDecl, Matcher<TypedefNameDecl>,
               InnerMatcher) {
   if (auto *Typedef = Node.getTypedefNameForAnonDecl())
@@ -2024,9 +2272,328 @@
   EXPECT_FALSE(NS->containsDecl(Spec));
 }
 
+struct ImportFunctionTemplateSpecializations : ASTImporterTestBase {};
+
+TEST_P(ImportFunctionTemplateSpecializations,
+       TUshouldNotContainFunctionTemplateImplicitInstantiation) {
+
+  Decl *FromTU = getTuDecl(
+      R"(
+      template<class T>
+      int f() { return 0; }
+      void foo() { f<int>(); }
+      )",
+      Lang_CXX, "input0.cc");
+
+  // Check that the function template instantiation is NOT the child of the TU.
+  auto Pattern = translationUnitDecl(
+      unless(has(functionDecl(hasName("f"), isTemplateInstantiation()))));
+  ASSERT_TRUE(MatchVerifier<Decl>{}.match(FromTU, Pattern));
+
+  auto *Foo = FirstDeclMatcher<FunctionDecl>().match(
+      FromTU, functionDecl(hasName("foo")));
+  ASSERT_TRUE(Import(Foo, Lang_CXX));
+
+  auto *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  EXPECT_TRUE(MatchVerifier<Decl>{}.match(ToTU, Pattern));
+}
+
+TEST_P(ImportFunctionTemplateSpecializations,
+       TUshouldNotContainFunctionTemplateExplicitInstantiation) {
+
+  Decl *FromTU = getTuDecl(
+      R"(
+      template<class T>
+      int f() { return 0; }
+      template int f<int>();
+      )",
+      Lang_CXX, "input0.cc");
+
+  // Check that the function template instantiation is NOT the child of the TU.
+  auto Instantiation = functionDecl(hasName("f"), isTemplateInstantiation());
+  auto Pattern = translationUnitDecl(unless(has(Instantiation)));
+  ASSERT_TRUE(MatchVerifier<Decl>{}.match(FromTU, Pattern));
+
+  ASSERT_TRUE(
+      Import(FirstDeclMatcher<Decl>().match(FromTU, Instantiation), Lang_CXX));
+
+  auto *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  EXPECT_TRUE(MatchVerifier<Decl>{}.match(ToTU, Pattern));
+}
+
+TEST_P(ImportFunctionTemplateSpecializations,
+       TUshouldContainFunctionTemplateSpecialization) {
+
+  Decl *FromTU = getTuDecl(
+      R"(
+      template<class T>
+      int f() { return 0; }
+      template <> int f<int>() { return 4; }
+      )",
+      Lang_CXX, "input0.cc");
+
+  // Check that the function template specialization is the child of the TU.
+  auto Specialization =
+      functionDecl(hasName("f"), isExplicitTemplateSpecialization());
+  auto Pattern = translationUnitDecl(has(Specialization));
+  ASSERT_TRUE(MatchVerifier<Decl>{}.match(FromTU, Pattern));
+
+  ASSERT_TRUE(
+      Import(FirstDeclMatcher<Decl>().match(FromTU, Specialization), Lang_CXX));
+
+  auto *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  EXPECT_TRUE(MatchVerifier<Decl>{}.match(ToTU, Pattern));
+}
+
+TEST_P(ImportFunctionTemplateSpecializations,
+       FunctionTemplateSpecializationRedeclChain) {
+
+  Decl *FromTU = getTuDecl(
+      R"(
+      template<class T>
+      int f() { return 0; }
+      template <> int f<int>() { return 4; }
+      )",
+      Lang_CXX, "input0.cc");
+
+  auto Spec = functionDecl(hasName("f"), isExplicitTemplateSpecialization(),
+                           hasParent(translationUnitDecl()));
+  auto *FromSpecD = FirstDeclMatcher<Decl>().match(FromTU, Spec);
+  {
+    auto *TU = FromTU;
+    auto *SpecD = FromSpecD;
+    auto *TemplateD = FirstDeclMatcher<FunctionTemplateDecl>().match(
+        TU, functionTemplateDecl());
+    auto *FirstSpecD = *(TemplateD->spec_begin());
+    ASSERT_EQ(SpecD, FirstSpecD);
+    ASSERT_TRUE(SpecD->getPreviousDecl());
+    ASSERT_FALSE(cast<FunctionDecl>(SpecD->getPreviousDecl())
+                     ->doesThisDeclarationHaveABody());
+  }
+
+  ASSERT_TRUE(Import(FromSpecD, Lang_CXX));
+
+  {
+    auto *TU = ToAST->getASTContext().getTranslationUnitDecl();
+    auto *SpecD = FirstDeclMatcher<Decl>().match(TU, Spec);
+    auto *TemplateD = FirstDeclMatcher<FunctionTemplateDecl>().match(
+        TU, functionTemplateDecl());
+    auto *FirstSpecD = *(TemplateD->spec_begin());
+    EXPECT_EQ(SpecD, FirstSpecD);
+    ASSERT_TRUE(SpecD->getPreviousDecl());
+    EXPECT_FALSE(cast<FunctionDecl>(SpecD->getPreviousDecl())
+                     ->doesThisDeclarationHaveABody());
+  }
+}
+
+TEST_P(ImportFunctionTemplateSpecializations,
+       MatchNumberOfFunctionTemplateSpecializations) {
+
+  Decl *FromTU = getTuDecl(
+      R"(
+      template <typename T> constexpr int f() { return 0; }
+      template <> constexpr int f<int>() { return 4; }
+      void foo() {
+        static_assert(f<char>() == 0, "");
+        static_assert(f<int>() == 4, "");
+      }
+      )",
+      Lang_CXX11, "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(
+      FromTU, functionDecl(hasName("foo")));
+
+  Import(FromD, Lang_CXX11);
+  auto *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  EXPECT_EQ(
+      DeclCounter<FunctionDecl>().match(FromTU, functionDecl(hasName("f"))),
+      DeclCounter<FunctionDecl>().match(ToTU, functionDecl(hasName("f"))));
+}
+
+TEST_P(ImportFunctionTemplateSpecializations,
+       ImportPrototypes) {
+  auto Pattern = functionDecl(hasName("f"), isExplicitTemplateSpecialization());
+  auto Code =
+      R"(
+      // Proto of the primary template.
+      template <class T>
+      void f();
+      // Proto of the specialization.
+      template <>
+      void f<int>();
+      )";
+
+  Decl *ImportedD;
+  {
+    Decl *FromTU = getTuDecl(Code, Lang_CXX, "input0.cc");
+    auto *FromD = LastDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+
+    ImportedD = Import(FromD, Lang_CXX);
+  }
+  {
+    Decl *FromTU = getTuDecl(Code, Lang_CXX, "input1.cc");
+    auto *FromD = LastDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    Import(FromD, Lang_CXX);
+  }
+
+  Decl *ToTU = ImportedD->getTranslationUnitDecl();
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_TRUE(ImportedD != To1);
+  EXPECT_FALSE(To0->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(To1->doesThisDeclarationHaveABody());
+  // Check that they are part of the same redecl chain.
+  EXPECT_EQ(To1->getCanonicalDecl(), To0->getCanonicalDecl());
+}
+
+TEST_P(ImportFunctionTemplateSpecializations, ImportDefinitions) {
+  auto Pattern = functionDecl(hasName("f"), isExplicitTemplateSpecialization());
+  auto Code =
+      R"(
+      // Proto of the primary template.
+      template <class T>
+      void f();
+      // Specialization and definition.
+      template <>
+      void f<int>() {}
+      )";
+
+  Decl *ImportedD;
+  {
+    Decl *FromTU = getTuDecl(Code, Lang_CXX, "input0.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    ImportedD = Import(FromD, Lang_CXX);
+  }
+  {
+    Decl *FromTU = getTuDecl(Code, Lang_CXX, "input1.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    Import(FromD, Lang_CXX);
+  }
+
+  Decl *ToTU = ImportedD->getTranslationUnitDecl();
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_TRUE(To0->doesThisDeclarationHaveABody());
+
+  auto *TemplateD = FirstDeclMatcher<FunctionTemplateDecl>().match(
+      ToTU, functionTemplateDecl());
+  auto *FirstSpecD = *(TemplateD->spec_begin());
+  EXPECT_EQ(FirstSpecD->getCanonicalDecl(), To0->getCanonicalDecl());
+}
+
+TEST_P(ImportFunctionTemplateSpecializations, PrototypeThenPrototype) {
+  auto Pattern = functionDecl(hasName("f"), isExplicitTemplateSpecialization());
+  auto Code =
+      R"(
+      // Proto of the primary template.
+      template <class T>
+      void f();
+      // Specialization proto.
+      template <>
+      void f<int>();
+      // Specialization proto.
+      template <>
+      void f<int>();
+      )";
+
+  Decl *ImportedD;
+  {
+    Decl *FromTU = getTuDecl(Code, Lang_CXX, "input0.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    ImportedD = Import(FromD, Lang_CXX);
+  }
+
+  Decl *ToTU = ImportedD->getTranslationUnitDecl();
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_TRUE(ImportedD != To1);
+  EXPECT_FALSE(To0->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(To1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(To1->getPreviousDecl(), To0);
+}
+
+TEST_P(ImportFunctionTemplateSpecializations, PrototypeThenDefinition) {
+  auto Pattern = functionDecl(hasName("f"), isExplicitTemplateSpecialization());
+  auto Code =
+      R"(
+      // Proto of the primary template.
+      template <class T>
+      void f();
+      // Specialization proto.
+      template <>
+      void f<int>();
+      // Specialization definition.
+      template <>
+      void f<int>() {}
+      )";
+
+  Decl *ImportedD;
+  {
+    Decl *FromTU = getTuDecl(Code, Lang_CXX, "input0.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    ImportedD = Import(FromD, Lang_CXX);
+  }
+
+  Decl *ToTU = ImportedD->getTranslationUnitDecl();
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_TRUE(ImportedD != To1);
+  EXPECT_FALSE(To0->doesThisDeclarationHaveABody());
+  EXPECT_TRUE(To1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(To1->getPreviousDecl(), To0);
+}
+
+TEST_P(ImportFunctionTemplateSpecializations, DefinitionThenPrototype) {
+  auto Pattern = functionDecl(hasName("f"), isExplicitTemplateSpecialization());
+  auto Code =
+      R"(
+      // Proto of the primary template.
+      template <class T>
+      void f();
+      // Specialization definition.
+      template <>
+      void f<int>() {}
+      // Specialization proto.
+      template <>
+      void f<int>();
+      )";
+
+  Decl *ImportedD;
+  {
+    Decl *FromTU = getTuDecl(Code, Lang_CXX, "input0.cc");
+    auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+    ImportedD = Import(FromD, Lang_CXX);
+  }
+
+  Decl *ToTU = ImportedD->getTranslationUnitDecl();
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, Pattern);
+  EXPECT_TRUE(ImportedD == To0);
+  EXPECT_TRUE(ImportedD != To1);
+  EXPECT_TRUE(To0->doesThisDeclarationHaveABody());
+  EXPECT_FALSE(To1->doesThisDeclarationHaveABody());
+  EXPECT_EQ(To1->getPreviousDecl(), To0);
+}
+
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, DeclContextTest,
                         ::testing::Values(ArgVector()), );
 
+INSTANTIATE_TEST_CASE_P(
+    ParameterizedTests, CanonicalRedeclChain,
+    ::testing::Values(ArgVector()),);
+
 auto DefaultTestValuesForRunOptions = ::testing::Values(
     ArgVector(),
     ArgVector{"-fdelayed-template-parsing"},
@@ -2048,5 +2615,12 @@
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ImportFunctions,
                         DefaultTestValuesForRunOptions, );
 
+INSTANTIATE_TEST_CASE_P(ParameterizedTests, ImportFriendFunctions,
+                        DefaultTestValuesForRunOptions, );
+
+INSTANTIATE_TEST_CASE_P(ParameterizedTests,
+                        ImportFunctionTemplateSpecializations,
+                        DefaultTestValuesForRunOptions, );
+
 } // end namespace ast_matchers
 } // end namespace clang