Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -4108,8 +4108,14 @@
       if (auto *FoundTemplate = dyn_cast<ClassTemplateDecl>(Found)) {
         if (IsStructuralMatch(D, FoundTemplate)) {
           // The class templates structurally match; call it the same template.
-          // FIXME: We may be filling in a forward declaration here. Handle
-          // this case!
+
+          // We found a forward declaration but the class to be imported has a
+          // definition.
+          // FIXME Add this forward declaration to the redeclaration chain.
+          if (D->isThisDeclarationADefinition() &&
+              !FoundTemplate->isThisDeclarationADefinition())
+            continue;
+
           Importer.Imported(D->getTemplatedDecl(), 
                             FoundTemplate->getTemplatedDecl());
           return Importer.Imported(D, FoundTemplate);
Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -1431,6 +1431,39 @@
       MatchVerifier<Decl>{}.match(To->getTranslationUnitDecl(), Pattern));
 }
 
+TEST_P(ASTImporterTestBase, ImportDefinitionOfClassTemplateAfterFwdDecl) {
+  {
+    Decl *FromTU = getTuDecl(
+        R"(
+            template <typename T>
+            struct B;
+            )",
+        Lang_CXX, "input0.cc");
+    auto *FromD = FirstDeclMatcher<ClassTemplateDecl>().match(
+        FromTU, classTemplateDecl(hasName("B")));
+
+    Import(FromD, Lang_CXX);
+  }
+
+  {
+    Decl *FromTU = getTuDecl(
+        R"(
+            template <typename T>
+            struct B {
+              void f();
+            };
+            )",
+        Lang_CXX, "input1.cc");
+    FunctionDecl *FromD = FirstDeclMatcher<FunctionDecl>().match(
+        FromTU, functionDecl(hasName("f")));
+    Import(FromD, Lang_CXX);
+    auto *FromCTD = FirstDeclMatcher<ClassTemplateDecl>().match(
+        FromTU, classTemplateDecl(hasName("B")));
+    auto *ToCTD = cast<ClassTemplateDecl>(Import(FromCTD, Lang_CXX));
+    EXPECT_TRUE(ToCTD->isThisDeclarationADefinition());
+  }
+}
+
 INSTANTIATE_TEST_CASE_P(
     ParameterizedTests, ASTImporterTestBase,
     ::testing::Values(ArgVector(), ArgVector{"-fdelayed-template-parsing"}),);