diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -5423,8 +5423,9 @@
 
   if (PrevDecl) {
     if (IsStructuralMatch(D, PrevDecl)) {
-      if (D->isThisDeclarationADefinition() && PrevDecl->getDefinition()) {
-        Importer.MapImported(D, PrevDecl->getDefinition());
+      CXXRecordDecl *PrevDefinition = PrevDecl->getDefinition();
+      if (D->isThisDeclarationADefinition() && PrevDefinition) {
+        Importer.MapImported(D, PrevDefinition);
         // Import those default field initializers which have been
         // instantiated in the "From" context, but not in the "To" context.
         for (auto *FromField : D->fields()) {
@@ -5446,7 +5447,7 @@
         //
         // Generally, ASTCommon.h/DeclUpdateKind enum gives a very good hint
         // what else could be fused during an AST merge.
-        return PrevDecl;
+        return PrevDefinition;
       }
     } else { // ODR violation.
       // FIXME HandleNameConflict
diff --git a/clang/unittests/AST/ASTImporterTest.cpp b/clang/unittests/AST/ASTImporterTest.cpp
--- a/clang/unittests/AST/ASTImporterTest.cpp
+++ b/clang/unittests/AST/ASTImporterTest.cpp
@@ -3104,6 +3104,25 @@
   EXPECT_TRUE(ToFun->hasBody());
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, MergeTemplateSpecWithForwardDecl) {
+  std::string ClassTemplate =
+      R"(
+      template<typename T>
+      struct X { int m; };
+      template<>
+      struct X<int> { int m; };
+      )";
+  // Append a forward decl for our template specialization.
+  getToTuDecl(ClassTemplate + "template<> struct X<int>;", Lang_CXX11);
+  Decl *FromTU = getTuDecl(ClassTemplate, Lang_CXX11);
+  auto *FromSpec = FirstDeclMatcher<ClassTemplateSpecializationDecl>().match(
+      FromTU, classTemplateSpecializationDecl(hasName("X"), isDefinition()));
+  auto *ImportedSpec = Import(FromSpec, Lang_CXX11);
+  // Check that our definition got merged with the existing definition.
+  EXPECT_TRUE(FromSpec->isThisDeclarationADefinition());
+  EXPECT_TRUE(ImportedSpec->isThisDeclarationADefinition());
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase,
        ODRViolationOfClassTemplateSpecializationsShouldBeReported) {
   std::string ClassTemplate =