Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -4961,15 +4961,14 @@
   return ToD;
 }
 
-// Returns the definition for a (forward) declaration of a ClassTemplateDecl, if
+// Returns the definition for a (forward) declaration of a TemplateDecl, if
 // it has any definition in the redecl chain.
-static ClassTemplateDecl *getDefinition(ClassTemplateDecl *D) {
-  CXXRecordDecl *ToTemplatedDef = D->getTemplatedDecl()->getDefinition();
+template <typename T> static auto getDefinition(T *D) -> T * {
+  auto *ToTemplatedDef = D->getTemplatedDecl()->getDefinition();
   if (!ToTemplatedDef)
     return nullptr;
-  ClassTemplateDecl *TemplateWithDef =
-      ToTemplatedDef->getDescribedClassTemplate();
-  return TemplateWithDef;
+  auto *TemplateWithDef = ToTemplatedDef->getDescribedTemplate();
+  return cast_or_null<T>(TemplateWithDef);
 }
 
 ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
@@ -5537,6 +5536,8 @@
   if (ToD)
     return ToD;
 
+  const FunctionTemplateDecl *FoundByLookup = nullptr;
+
   // 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()) {
@@ -5546,19 +5547,22 @@
       if (!FoundDecl->isInIdentifierNamespace(IDNS))
         continue;
 
-      if (auto *FoundFunction =
-          dyn_cast<FunctionTemplateDecl>(FoundDecl)) {
-        if (FoundFunction->hasExternalFormalLinkage() &&
+      if (auto *FoundTemplate = dyn_cast<FunctionTemplateDecl>(FoundDecl)) {
+        if (FoundTemplate->hasExternalFormalLinkage() &&
             D->hasExternalFormalLinkage()) {
-          if (IsStructuralMatch(D, FoundFunction)) {
-            Importer.MapImported(D, FoundFunction);
-            // FIXME: Actually try to merge the body and other attributes.
-            return FoundFunction;
+          if (IsStructuralMatch(D, FoundTemplate)) {
+            FunctionTemplateDecl *TemplateWithDef =
+                getDefinition(FoundTemplate);
+            if (D->isThisDeclarationADefinition() && TemplateWithDef) {
+              return Importer.MapImported(D, TemplateWithDef);
+            }
+            FoundByLookup = FoundTemplate;
+            break;
           }
-        }
-      }
-      // TODO: handle conflicting names
-    }
+          // TODO: handle conflicting names
+        } // linkage
+      }   // template
+    }     // for
   }
 
   auto ParamsOrErr = ImportTemplateParameterList(
@@ -5576,10 +5580,23 @@
     return ToFunc;
 
   TemplatedFD->setDescribedFunctionTemplate(ToFunc);
+
   ToFunc->setAccess(D->getAccess());
   ToFunc->setLexicalDeclContext(LexicalDC);
-
   LexicalDC->addDeclInternal(ToFunc);
+
+  if (FoundByLookup) {
+    auto *Recent =
+        const_cast<FunctionTemplateDecl *>(FoundByLookup->getMostRecentDecl());
+    if (!TemplatedFD->getPreviousDecl()) {
+      auto *PrevTemplated =
+          FoundByLookup->getTemplatedDecl()->getMostRecentDecl();
+      if (TemplatedFD != PrevTemplated)
+        TemplatedFD->setPreviousDecl(PrevTemplated);
+    }
+    ToFunc->setPreviousDecl(Recent);
+  }
+
   return ToFunc;
 }
 
Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -4163,9 +4163,8 @@
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(
     RedeclChain, Variable, ,
     PrototypeShouldBeImportedAsAPrototypeWhenThereIsNoDefinition)
-// FIXME Enable this test, once we import function templates chains correctly.
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(
-    RedeclChain, FunctionTemplate, DISABLED_,
+    RedeclChain, FunctionTemplate, ,
     PrototypeShouldBeImportedAsAPrototypeWhenThereIsNoDefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(
     RedeclChain, ClassTemplate, ,
@@ -4180,9 +4179,8 @@
     RedeclChain, Class, , DefinitionShouldBeImportedAsADefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(
     RedeclChain, Variable, , DefinitionShouldBeImportedAsADefinition)
-// FIXME Enable this test, once we import function templates chains correctly.
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(
-    RedeclChain, FunctionTemplate, DISABLED_,
+    RedeclChain, FunctionTemplate, ,
     DefinitionShouldBeImportedAsADefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(
     RedeclChain, ClassTemplate, , DefinitionShouldBeImportedAsADefinition)
@@ -4196,9 +4194,7 @@
                                         ImportPrototypeAfterImportedPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportPrototypeAfterImportedPrototype)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
                                         ImportPrototypeAfterImportedPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportPrototypeAfterImportedPrototype)
@@ -4212,9 +4208,7 @@
                                         ImportDefinitionAfterImportedPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportDefinitionAfterImportedPrototype)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
                                         ImportDefinitionAfterImportedPrototype)
 // FIXME This does not pass, possible error with ClassTemplate import.
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, DISABLED_,
@@ -4229,9 +4223,7 @@
                                         ImportPrototypeAfterImportedDefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportPrototypeAfterImportedDefinition)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
                                         ImportPrototypeAfterImportedDefinition)
 // FIXME This does not pass, possible error with ClassTemplate import.
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, DISABLED_,
@@ -4244,9 +4236,8 @@
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Class, , ImportPrototypes)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportPrototypes)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_, ImportPrototypes)
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
+                                        ImportPrototypes)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportPrototypes)
 // FIXME This does not pass, possible error with Spec import.
@@ -4259,9 +4250,8 @@
                                         ImportDefinitions)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportDefinitions)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_, ImportDefinitions)
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
+                                        ImportDefinitions)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportDefinitions)
 // FIXME This does not pass, possible error with Spec import.
@@ -4274,9 +4264,7 @@
                                         ImportDefinitionThenPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportDefinitionThenPrototype)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
                                         ImportDefinitionThenPrototype)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportDefinitionThenPrototype)
@@ -4291,9 +4279,7 @@
                                         ImportPrototypeThenDefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportPrototypeThenDefinition)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
                                         ImportPrototypeThenDefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, ClassTemplate, ,
                                         ImportPrototypeThenDefinition)
@@ -4306,9 +4292,7 @@
                                         WholeRedeclChainIsImportedAtOnce)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         WholeRedeclChainIsImportedAtOnce)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
                                         WholeRedeclChainIsImportedAtOnce)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplateSpec, ,
                                         WholeRedeclChainIsImportedAtOnce)
@@ -4317,9 +4301,7 @@
                                         ImportPrototypeThenProtoAndDefinition)
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, Variable, ,
                                         ImportPrototypeThenProtoAndDefinition)
-// FIXME Enable this test, once we import function templates chains correctly.
-ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate,
-                                        DISABLED_,
+ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplate, ,
                                         ImportPrototypeThenProtoAndDefinition)
 // FIXME This does not pass, possible error with Spec import.
 ASTIMPORTER_INSTANTIATE_TYPED_TEST_CASE(RedeclChain, FunctionTemplateSpec,
@@ -5412,9 +5394,7 @@
       FromTU, functionTemplateDecl(hasName("foo")));
   auto *Imported = Import(FromFoo, Lang_CXX);
 
-  // FIXME Currently chains of FunctionTemplateDecls are not implemented.
-  //EXPECT_EQ(Imported->getPreviousDecl(), Friend);
-  EXPECT_EQ(Imported, Friend);
+  EXPECT_EQ(Imported->getPreviousDecl(), Friend);
 }
 
 INSTANTIATE_TEST_CASE_P(ParameterizedTests, ASTImporterLookupTableTest,