Index: include/clang/ASTMatchers/ASTMatchers.h
===================================================================
--- include/clang/ASTMatchers/ASTMatchers.h
+++ include/clang/ASTMatchers/ASTMatchers.h
@@ -420,6 +420,25 @@
     Decl, ClassTemplateSpecializationDecl>
     classTemplateSpecializationDecl;
 
+/// Matches C++ class template partial specializations.
+///
+/// Given
+/// \code
+///   template<class T1, class T2, int I>
+///   class A {};
+///
+///   template<class T, int I>
+///   class A<T, T*, I> {};
+///
+///   template<>
+///   class A<int, int, 1> {};
+/// \endcode
+/// classTemplatePartialSpecializationDecl()
+///   matches the specialization \c A<T,T*,I> but not \c A<int,int,1>
+extern const internal::VariadicDynCastAllOfMatcher<
+    Decl, ClassTemplatePartialSpecializationDecl>
+    classTemplatePartialSpecializationDecl;
+
 /// Matches declarator declarations (field, variable, function
 /// and non-type template parameter declarations).
 ///
Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -2877,6 +2877,22 @@
       if (Importer.IsStructurallyEquivalent(D->getType(),
                                             FoundField->getType())) {
         Importer.MapImported(D, FoundField);
+        // In case of a FieldDecl of a ClassTemplateSpecializationDecl, the
+        // initializer of a FieldDecl might not had been instantiated in the
+        // "To" context.  However, the "From" context might instantiated that,
+        // thus we have to merge that.
+        if (Expr *FromInitializer = D->getInClassInitializer()) {
+          // We don't have yet the initializer set.
+          if (FoundField->hasInClassInitializer() &&
+              !FoundField->getInClassInitializer()) {
+            Expr *ToInitializer = Importer.Import(FromInitializer);
+            if (!ToInitializer)
+              // We can't return a nullptr here,
+              // since we already mapped D as imported.
+              return FoundField;
+            FoundField->setInClassInitializer(ToInitializer);
+          }
+        }
         return FoundField;
       }
 
@@ -4518,27 +4534,49 @@
 
   // Try to find an existing specialization with these template arguments.
   void *InsertPos = nullptr;
-  ClassTemplateSpecializationDecl *D2
-    = ClassTemplate->findSpecialization(TemplateArgs, InsertPos);
-  if (D2) {
-    // We already have a class template specialization with these template
-    // arguments.
+  ClassTemplateSpecializationDecl *D2 = nullptr;
+  ClassTemplatePartialSpecializationDecl *PartialSpec =
+            dyn_cast<ClassTemplatePartialSpecializationDecl>(D);
+  if (PartialSpec)
+    D2 = ClassTemplate->findPartialSpecialization(TemplateArgs, InsertPos);
+  else
+    D2 = ClassTemplate->findSpecialization(TemplateArgs, InsertPos);
+  ClassTemplateSpecializationDecl * const PrevDecl = D2;
+  RecordDecl *FoundDef = D2 ? D2->getDefinition() : nullptr;
+  if (FoundDef) {
+    if (!D->isCompleteDefinition()) {
+      // The "From" translation unit only had a forward declaration; call it
+      // the same declaration.
+      // TODO Handle the redecl chain properly!
+      return Importer.MapImported(D, FoundDef);
+    }
 
-    // FIXME: Check for specialization vs. instantiation errors.
+    if (IsStructuralMatch(D, FoundDef)) {
 
-    if (RecordDecl *FoundDef = D2->getDefinition()) {
-      if (!D->isCompleteDefinition() || IsStructuralMatch(D, FoundDef)) {
-        // The record types structurally match, or the "from" translation
-        // unit only had a forward declaration anyway; call it the same
-        // function.
-        return Importer.MapImported(D, FoundDef);
-      }
+      Importer.MapImported(D, FoundDef);
+
+      // Import those those default field initializers which have been
+      // instantiated in the "From" context, but not in the "To" context.
+      for (auto *FromField : D->fields())
+        Importer.Import(FromField);
+
+      // Import those methods which have been instantiated in the
+      // "From" context, but not in the "To" context.
+      for (CXXMethodDecl *FromM : D->methods())
+        Importer.Import(FromM);
+
+      // TODO  Import instantiated default arguments.
+      //
+      // Generally, ASTCommon.h/DeclUpdateKind enum gives a very good hint what
+      // else could be fused during an AST merge.
+
+      return FoundDef;
     }
-  } else {
-    // Create a new specialization.
-    if (auto *PartialSpec =
-            dyn_cast<ClassTemplatePartialSpecializationDecl>(D)) {
-      // Import TemplateArgumentListInfo
+  } else { // We either couldn't find any previous specialization in the "To"
+           // context,  or we found one but without definition.  Let's create a
+           // new specialization and register that at the class template.
+    if (PartialSpec) {
+      // Import TemplateArgumentListInfo.
       TemplateArgumentListInfo ToTAInfo;
       const auto &ASTTemplateArgs = *PartialSpec->getTemplateArgsAsWritten();
       if (ImportTemplateArgumentListInfo(ASTTemplateArgs, ToTAInfo))
@@ -4559,21 +4597,32 @@
               D2, D, Importer.getToContext(), D->getTagKind(), DC, StartLoc,
               IdLoc, ToTPList, ClassTemplate,
               llvm::makeArrayRef(TemplateArgs.data(), TemplateArgs.size()),
-              ToTAInfo, CanonInjType, nullptr))
+              ToTAInfo, CanonInjType,
+              cast_or_null<ClassTemplatePartialSpecializationDecl>(PrevDecl)))
         return D2;
 
-    } else {
+      // Update InsertPos, because preceding import calls may have invalidated
+      // it by adding new specializations.
+      if (!ClassTemplate->findPartialSpecialization(TemplateArgs, InsertPos))
+        // Add this partial specialization to the class template.
+        ClassTemplate->AddPartialSpecialization(
+            cast<ClassTemplatePartialSpecializationDecl>(D2), InsertPos);
+
+    } else { // Not a partial specialization.
       if (GetImportedOrCreateDecl(
               D2, D, Importer.getToContext(), D->getTagKind(), DC, StartLoc,
-              IdLoc, ClassTemplate, TemplateArgs, /*PrevDecl=*/nullptr))
+              IdLoc, ClassTemplate, TemplateArgs, PrevDecl))
         return D2;
+
+      // Update InsertPos, because preceding import calls may have invalidated
+      // it by adding new specializations.
+      if (!ClassTemplate->findSpecialization(TemplateArgs, InsertPos))
+        // Add this specialization to the class template.
+        ClassTemplate->AddSpecialization(D2, InsertPos);
     }
 
     D2->setSpecializationKind(D->getSpecializationKind());
 
-    // Add this specialization to the class template.
-    ClassTemplate->AddSpecialization(D2, InsertPos);
-
     // Import the qualifier, if any.
     D2->setQualifierInfo(Importer.Import(D->getQualifierLoc()));
 
Index: lib/ASTMatchers/ASTMatchersInternal.cpp
===================================================================
--- lib/ASTMatchers/ASTMatchersInternal.cpp
+++ lib/ASTMatchers/ASTMatchersInternal.cpp
@@ -573,6 +573,9 @@
 const internal::VariadicDynCastAllOfMatcher<Decl,
                                             ClassTemplateSpecializationDecl>
     classTemplateSpecializationDecl;
+const internal::VariadicDynCastAllOfMatcher<
+    Decl, ClassTemplatePartialSpecializationDecl>
+    classTemplatePartialSpecializationDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, DeclaratorDecl>
     declaratorDecl;
 const internal::VariadicDynCastAllOfMatcher<Decl, ParmVarDecl> parmVarDecl;
Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -2683,6 +2683,263 @@
   EXPECT_EQ(FromIndex, 3u);
 }
 
+TEST_P(ASTImporterTestBase, MergeFieldDeclsOfClassTemplateSpecialization) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {
+          int a{0}; // FieldDecl with InitListExpr
+          X(char) : a(3) {}     // (1)
+          X(int) {}             // (2)
+      };
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+      R"(
+      void foo() {
+          // ClassTemplateSpec with ctor (1): FieldDecl without InitlistExpr
+          X<char> xc('c');
+      }
+      )", Lang_CXX11);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  // FieldDecl without InitlistExpr:
+  auto *ToField = *ToSpec->field_begin();
+  ASSERT_TRUE(ToField);
+  ASSERT_FALSE(ToField->getInClassInitializer());
+  Decl *FromTU = getTuDecl(ClassTemplate +
+      R"(
+      void bar() {
+          // ClassTemplateSpec with ctor (2): FieldDecl WITH InitlistExpr
+          X<char> xc(1);
+      }
+      )", Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  // FieldDecl with InitlistExpr:
+  auto *FromField = *FromSpec->field_begin();
+  ASSERT_TRUE(FromField);
+  ASSERT_TRUE(FromField->getInClassInitializer());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  ASSERT_TRUE(ImportedSpec);
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  // After the import, the FieldDecl has to be merged, thus it should have the
+  // InitListExpr.
+  EXPECT_TRUE(ToField->getInClassInitializer());
+}
+
+TEST_P(ASTImporterTestBase, MergeFunctionOfClassTemplateSpecialization) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {
+        void f() {}
+        void g() {}
+      };
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+      R"(
+      void foo() {
+          X<char> x;
+          x.f();
+      }
+      )", Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate +
+      R"(
+      void bar() {
+          X<char> x;
+          x.g();
+      }
+      )", Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  auto FunPattern = functionDecl(hasName("g"),
+                         hasParent(classTemplateSpecializationDecl()));
+  auto *FromFun =
+      FirstDeclMatcher<FunctionDecl>().match(FromTU, FunPattern);
+  auto *ToFun =
+      FirstDeclMatcher<FunctionDecl>().match(ToTU, FunPattern);
+  ASSERT_TRUE(FromFun->hasBody());
+  ASSERT_FALSE(ToFun->hasBody());
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  ASSERT_TRUE(ImportedSpec);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_TRUE(ToFun->hasBody());
+}
+
+TEST_P(ASTImporterTestBase,
+       ODRViolationOfClassTemplateSpecializationsShouldBeReported) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {};
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+                               R"(
+      template <>
+      struct X<char> {
+          int a;
+      };
+      void foo() {
+          X<char> x;
+      }
+      )",
+                           Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate +
+                               R"(
+      template <>
+      struct X<char> {
+          int b;
+      };
+      void foo() {
+          X<char> x;
+      }
+      )",
+                           Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+
+  // We expect one (ODR) warning during the import.
+  EXPECT_EQ(1u, ToTU->getASTContext().getDiagnostics().getNumWarnings());
+
+  // The second specialization is different from the first, thus it violates
+  // ODR, consequently we expect to keep the first specialization only, which is
+  // already in the "To" context.
+  EXPECT_TRUE(ImportedSpec);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_EQ(1u, DeclCounter<ClassTemplateSpecializationDecl>().match(
+                    ToTU, classTemplateSpecializationDecl()));
+}
+
+TEST_P(ASTImporterTestBase, MergeCtorOfClassTemplateSpecialization) {
+  std::string ClassTemplate =
+      R"(
+      template <typename T>
+      struct X {
+          X(char) {}
+          X(int) {}
+      };
+      )";
+  Decl *ToTU = getToTuDecl(ClassTemplate +
+      R"(
+      void foo() {
+          X<char> x('c');
+      }
+      )", Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate +
+      R"(
+      void bar() {
+          X<char> x(1);
+      }
+      )", Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X")));
+  // Match the void(int) ctor.
+  auto CtorPattern =
+      cxxConstructorDecl(hasParameter(0, varDecl(hasType(asString("int")))),
+                         hasParent(classTemplateSpecializationDecl()));
+  auto *FromCtor =
+      FirstDeclMatcher<CXXConstructorDecl>().match(FromTU, CtorPattern);
+  auto *ToCtor =
+      FirstDeclMatcher<CXXConstructorDecl>().match(ToTU, CtorPattern);
+  ASSERT_TRUE(FromCtor->hasBody());
+  ASSERT_FALSE(ToCtor->hasBody());
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  ASSERT_TRUE(ImportedSpec);
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl(hasName("X")));
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_TRUE(ToCtor->hasBody());
+}
+
+TEST_P(ASTImporterTestBase,
+       ClassTemplatePartialSpecializationsShouldNotBeDuplicated) {
+  auto Code =
+      R"(
+    // primary template
+    template<class T1, class T2, int I>
+    class A {};
+
+    // partial specialization
+    template<class T, int I>
+    class A<T, T*, I> {};
+    )";
+  Decl *ToTU = getToTuDecl(Code, Lang_CXX11);
+  Decl *FromTU = getTuDecl(Code, Lang_CXX11);
+  auto *FromSpec =
+      FirstDeclMatcher<ClassTemplatePartialSpecializationDecl>().match(
+          FromTU, classTemplatePartialSpecializationDecl());
+  auto *ToSpec =
+      FirstDeclMatcher<ClassTemplatePartialSpecializationDecl>().match(
+          ToTU, classTemplatePartialSpecializationDecl());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_EQ(1u, DeclCounter<ClassTemplatePartialSpecializationDecl>().match(
+                    ToTU, classTemplatePartialSpecializationDecl()));
+}
+
+TEST_P(ASTImporterTestBase, ClassTemplateSpecializationsShouldNotBeDuplicated) {
+  auto Code =
+      R"(
+    // primary template
+    template<class T1, class T2, int I>
+    class A {};
+
+    // full specialization
+    template<>
+    class A<int, int, 1> {};
+    )";
+  Decl *ToTU = getToTuDecl(Code, Lang_CXX11);
+  Decl *FromTU = getTuDecl(Code, Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl());
+  auto *ToSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      ToTU, classTemplateSpecializationDecl());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  EXPECT_EQ(ImportedSpec, ToSpec);
+  EXPECT_EQ(1u, DeclCounter<ClassTemplateSpecializationDecl>().match(
+                   ToTU, classTemplateSpecializationDecl()));
+}
+
+TEST_P(ASTImporterTestBase, ClassTemplateFullAndPartialSpecsShouldNotBeMixed) {
+  std::string PrimaryTemplate =
+      R"(
+    template<class T1, class T2, int I>
+    class A {};
+    )";
+  auto PartialSpec =
+      R"(
+    template<class T, int I>
+    class A<T, T*, I> {};
+    )";
+  auto FullSpec =
+      R"(
+    template<>
+    class A<int, int, 1> {};
+    )";
+  Decl *ToTU = getToTuDecl(PrimaryTemplate + FullSpec, Lang_CXX11);
+  Decl *FromTU = getTuDecl(PrimaryTemplate + PartialSpec, Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl());
+
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  EXPECT_TRUE(ImportedSpec);
+  // Check the number of partial specializations.
+  EXPECT_EQ(1u, DeclCounter<ClassTemplatePartialSpecializationDecl>().match(
+                    ToTU, classTemplatePartialSpecializationDecl()));
+  // Check the number of full specializations.
+  EXPECT_EQ(1u, DeclCounter<ClassTemplateSpecializationDecl>().match(
+                    ToTU, classTemplateSpecializationDecl(
+                              unless(classTemplatePartialSpecializationDecl()))));
+}
+
 struct DeclContextTest : ASTImporterTestBase {};
 
 TEST_P(DeclContextTest, removeDeclOfClassTemplateSpecialization) {