Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -2275,6 +2275,205 @@
   EXPECT_EQ(ImportedD1->getPreviousDecl(), ImportedD);
 }
 
+TEST_P(ImportFriendFunctions, Lookup) {
+  auto FunctionPattern = functionDecl(hasName("f"));
+  auto ClassPattern = cxxRecordDecl(hasName("X"));
+
+  Decl *FromTU =
+      getTuDecl("struct X { friend void f(); };", Lang_CXX, "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  ASSERT_TRUE(FromD->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_FALSE(FromD->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  {
+    auto FromName = FromD->getDeclName();
+    auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
+    auto LookupRes = Class->noload_lookup(FromName);
+    ASSERT_EQ(LookupRes.size(), 0u);
+    LookupRes = cast<TranslationUnitDecl>(FromTU)->noload_lookup(FromName);
+    ASSERT_EQ(LookupRes.size(), 1u);
+  }
+
+  auto *ToD = cast<FunctionDecl>(Import(FromD, Lang_CXX));
+  auto ToName = ToD->getDeclName();
+
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
+  auto LookupRes = Class->noload_lookup(ToName);
+  EXPECT_EQ(LookupRes.size(), 0u);
+  LookupRes = ToTU->noload_lookup(ToName);
+  EXPECT_EQ(LookupRes.size(), 1u);
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, FunctionPattern), 1u);
+  auto *To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  EXPECT_TRUE(To0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_FALSE(To0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+}
+
+TEST_P(ImportFriendFunctions, DISABLED_LookupWithProtoAfter) {
+  auto FunctionPattern = functionDecl(hasName("f"));
+  auto ClassPattern = cxxRecordDecl(hasName("X"));
+
+  TranslationUnitDecl *FromTU =
+      getTuDecl("struct X { friend void f(); };"
+                // This proto decl makes f available to normal
+                // lookup, otherwise it is hidden.
+                // Normal C++ lookup (implemented in
+                // `clang::Sema::CppLookupName()` and in `LookupDirect()`)
+                // returns the found `NamedDecl` only if the set IDNS is matched
+                "void f();",
+                Lang_CXX, "input0.cc");
+  auto *From0 = FirstDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  auto *From1 = LastDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  ASSERT_TRUE(From0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_FALSE(From0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_FALSE(From1->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_TRUE(From1->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  {
+    auto Name = From0->getDeclName();
+    auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
+    auto LookupRes = Class->noload_lookup(Name);
+    ASSERT_EQ(LookupRes.size(), 0u);
+    LookupRes = FromTU->noload_lookup(Name);
+    ASSERT_EQ(LookupRes.size(), 1u);
+  }
+
+  auto *To0 = cast<FunctionDecl>(Import(From0, Lang_CXX));
+  auto Name = To0->getDeclName();
+
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
+  auto LookupRes = Class->noload_lookup(Name);
+  EXPECT_EQ(LookupRes.size(), 0u);
+  LookupRes = ToTU->noload_lookup(Name);
+  // Test is disabled because this result is 2.
+  EXPECT_EQ(LookupRes.size(), 1u);
+
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, FunctionPattern), 2u);
+  To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  EXPECT_TRUE(To0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_FALSE(To0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_FALSE(To1->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_TRUE(To1->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+}
+
+TEST_P(ImportFriendFunctions, LookupWithProtoBefore) {
+  auto FunctionPattern = functionDecl(hasName("f"));
+  auto ClassPattern = cxxRecordDecl(hasName("X"));
+
+  TranslationUnitDecl *FromTU =
+      getTuDecl("void f();"
+                "struct X { friend void f(); };",
+                Lang_CXX, "input0.cc");
+  auto *From0 = FirstDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  auto *From1 = LastDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  ASSERT_FALSE(From0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_TRUE(From0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_TRUE(From1->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_TRUE(From1->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  {
+    auto Name = From0->getDeclName();
+    auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
+    auto LookupRes = Class->noload_lookup(Name);
+    ASSERT_EQ(LookupRes.size(), 0u);
+    LookupRes = FromTU->noload_lookup(Name);
+    ASSERT_EQ(LookupRes.size(), 1u);
+  }
+
+  auto *To0 = cast<FunctionDecl>(Import(From0, Lang_CXX));
+  auto Name = To0->getDeclName();
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+
+  auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
+  auto LookupRes = Class->noload_lookup(Name);
+  EXPECT_EQ(LookupRes.size(), 0u);
+  LookupRes = ToTU->noload_lookup(Name);
+  EXPECT_EQ(LookupRes.size(), 1u);
+
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, FunctionPattern), 2u);
+  To0 = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  auto *To1 = LastDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  EXPECT_FALSE(To0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_TRUE(To0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_TRUE(To1->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_TRUE(To1->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+}
+
+TEST_P(ImportFriendFunctions, ImportFriendChangesLookup) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  TranslationUnitDecl *FromTU0 = getTuDecl("void f();", Lang_CXX, "input0.cc");
+  auto *FromD0 = FirstDeclMatcher<FunctionDecl>().match(FromTU0, Pattern);
+  TranslationUnitDecl *FromTU1 =
+      getTuDecl("class X { friend void f(); };", Lang_CXX, "input1.cc");
+  auto *FromD1 = FirstDeclMatcher<FunctionDecl>().match(FromTU1, Pattern);
+  auto FromName0 = FromD0->getDeclName();
+  auto FromName1 = FromD1->getDeclName();
+
+  ASSERT_TRUE(FromD0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_FALSE(FromD0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_FALSE(FromD1->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_TRUE(FromD1->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  auto LookupRes = FromTU0->noload_lookup(FromName0);
+  ASSERT_EQ(LookupRes.size(), 1u);
+  LookupRes = FromTU1->noload_lookup(FromName1);
+  ASSERT_EQ(LookupRes.size(), 1u);
+
+  auto *ToD0 = cast<FunctionDecl>(Import(FromD0, Lang_CXX));
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  auto ToName = ToD0->getDeclName();
+  EXPECT_TRUE(ToD0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_FALSE(ToD0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  LookupRes = ToTU->noload_lookup(ToName);
+  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
+  
+  auto *ToD1 = cast<FunctionDecl>(Import(FromD1, Lang_CXX));
+  LookupRes = ToTU->noload_lookup(ToName);
+  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+
+  EXPECT_TRUE(ToD0->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_FALSE(ToD0->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+
+  EXPECT_TRUE(ToD1->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_TRUE(ToD1->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+}
+
+TEST_P(ImportFriendFunctions, ImportFriendList) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  TranslationUnitDecl *FromTU = getTuDecl(
+      "struct X { friend void f(); };"
+      "void f();",
+      Lang_CXX, "input0.cc");
+  auto *FromD = FirstDeclMatcher<FunctionDecl>().match(FromTU, Pattern);
+  {
+    auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(
+        FromTU, cxxRecordDecl(hasName("X")));
+    auto *Friend = FirstDeclMatcher<FriendDecl>().match(FromTU, friendDecl());
+    auto Friends = Class->friends();
+    unsigned int FrN = 0;
+    for (auto Fr : Friends) {
+      ASSERT_EQ(Fr, Friend);
+      ++FrN;
+    }
+    ASSERT_EQ(FrN, 1u);
+  }
+  Import(FromD, Lang_CXX);
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  auto *Class = FirstDeclMatcher<CXXRecordDecl>().match(
+      ToTU, cxxRecordDecl(hasName("X")));
+  auto *Friend = FirstDeclMatcher<FriendDecl>().match(ToTU, friendDecl());
+  auto Friends = Class->friends();
+  unsigned int FrN = 0;
+  for (auto Fr : Friends) {
+    EXPECT_EQ(Fr, Friend);
+    ++FrN;
+  }
+  EXPECT_EQ(FrN, 1u);
+}
+
 AST_MATCHER_P(TagDecl, hasTypedefForAnonDecl, Matcher<TypedefNameDecl>,
               InnerMatcher) {
   if (auto *Typedef = Node.getTypedefNameForAnonDecl())