Index: unittests/AST/ASTImporterTest.cpp
===================================================================
--- unittests/AST/ASTImporterTest.cpp
+++ unittests/AST/ASTImporterTest.cpp
@@ -2274,6 +2274,211 @@
   EXPECT_EQ(ImportedD1->getPreviousDecl(), ImportedD);
 }
 
+TEST_P(ImportFriendFunctions, Lookup) {
+  auto FunctionPattern = functionDecl(hasName("f"));
+  auto ClassPattern = cxxRecordDecl(hasName("X"));
+
+  TranslationUnitDecl *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 = 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 *FromFriend =
+      FirstDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  auto *FromNormal =
+      LastDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  ASSERT_TRUE(FromFriend->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_FALSE(FromFriend->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_FALSE(FromNormal->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_TRUE(FromNormal->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+
+  auto FromName = FromFriend->getDeclName();
+  auto *FromClass =
+      FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
+  auto LookupRes = FromClass->noload_lookup(FromName);
+  ASSERT_EQ(LookupRes.size(), 0u);
+  LookupRes = FromTU->noload_lookup(FromName);
+  ASSERT_EQ(LookupRes.size(), 1u);
+
+  auto *ToFriend = cast<FunctionDecl>(Import(FromFriend, Lang_CXX));
+  auto ToName = ToFriend->getDeclName();
+
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  auto *ToClass = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
+  LookupRes = ToClass->noload_lookup(ToName);
+  EXPECT_EQ(LookupRes.size(), 0u);
+  LookupRes = ToTU->noload_lookup(ToName);
+  // Test is disabled because this result is 2.
+  EXPECT_EQ(LookupRes.size(), 1u);
+
+  ASSERT_EQ(DeclCounter<FunctionDecl>().match(ToTU, FunctionPattern), 2u);
+  ToFriend = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  auto *ToNormal = LastDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  EXPECT_TRUE(ToFriend->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_FALSE(ToFriend->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_FALSE(ToNormal->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_TRUE(ToNormal->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 *FromNormal =
+      FirstDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  auto *FromFriend =
+      LastDeclMatcher<FunctionDecl>().match(FromTU, FunctionPattern);
+  ASSERT_FALSE(FromNormal->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_TRUE(FromNormal->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_TRUE(FromFriend->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_TRUE(FromFriend->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+
+  auto FromName = FromNormal->getDeclName();
+  auto *FromClass =
+      FirstDeclMatcher<CXXRecordDecl>().match(FromTU, ClassPattern);
+  auto LookupRes = FromClass->noload_lookup(FromName);
+  ASSERT_EQ(LookupRes.size(), 0u);
+  LookupRes = FromTU->noload_lookup(FromName);
+  ASSERT_EQ(LookupRes.size(), 1u);
+
+  auto *ToNormal = cast<FunctionDecl>(Import(FromNormal, Lang_CXX));
+  auto ToName = ToNormal->getDeclName();
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+
+  auto *ToClass = FirstDeclMatcher<CXXRecordDecl>().match(ToTU, ClassPattern);
+  LookupRes = ToClass->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), 2u);
+  ToNormal = FirstDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  auto *ToFriend = LastDeclMatcher<FunctionDecl>().match(ToTU, FunctionPattern);
+  EXPECT_FALSE(ToNormal->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_TRUE(ToNormal->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_TRUE(ToFriend->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  EXPECT_TRUE(ToFriend->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+}
+
+TEST_P(ImportFriendFunctions, ImportFriendChangesLookup) {
+  auto Pattern = functionDecl(hasName("f"));
+
+  TranslationUnitDecl *FromNormalTU =
+      getTuDecl("void f();", Lang_CXX, "input0.cc");
+  auto *FromNormalF =
+      FirstDeclMatcher<FunctionDecl>().match(FromNormalTU, Pattern);
+  TranslationUnitDecl *FromFriendTU =
+      getTuDecl("class X { friend void f(); };", Lang_CXX, "input1.cc");
+  auto *FromFriendF =
+      FirstDeclMatcher<FunctionDecl>().match(FromFriendTU, Pattern);
+  auto FromNormalName = FromNormalF->getDeclName();
+  auto FromFriendName = FromFriendF->getDeclName();
+
+  ASSERT_TRUE(FromNormalF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_FALSE(FromNormalF->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  ASSERT_FALSE(FromFriendF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  ASSERT_TRUE(FromFriendF->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  auto LookupRes = FromNormalTU->noload_lookup(FromNormalName);
+  ASSERT_EQ(LookupRes.size(), 1u);
+  LookupRes = FromFriendTU->noload_lookup(FromFriendName);
+  ASSERT_EQ(LookupRes.size(), 1u);
+
+  auto *ToNormalF = cast<FunctionDecl>(Import(FromNormalF, Lang_CXX));
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  auto ToName = ToNormalF->getDeclName();
+  EXPECT_TRUE(ToNormalF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_FALSE(ToNormalF->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+  LookupRes = ToTU->noload_lookup(ToName);
+  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 1u);
+  
+  auto *ToFriendF = cast<FunctionDecl>(Import(FromFriendF, Lang_CXX));
+  LookupRes = ToTU->noload_lookup(ToName);
+  EXPECT_EQ(LookupRes.size(), 1u);
+  EXPECT_EQ(DeclCounter<FunctionDecl>().match(ToTU, Pattern), 2u);
+
+  EXPECT_TRUE(ToNormalF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_FALSE(ToNormalF->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+
+  EXPECT_TRUE(ToFriendF->isInIdentifierNamespace(Decl::IDNS_Ordinary));
+  EXPECT_TRUE(ToFriendF->isInIdentifierNamespace(Decl::IDNS_OrdinaryFriend));
+}
+
+TEST_P(ImportFriendFunctions, ImportFriendList) {
+  TranslationUnitDecl *FromTU = getTuDecl(
+      "struct X { friend void f(); };"
+      "void f();",
+      Lang_CXX, "input0.cc");
+  auto *FromFriendF = FirstDeclMatcher<FunctionDecl>().match(
+      FromTU, functionDecl(hasName("f")));
+
+  auto *FromClass = FirstDeclMatcher<CXXRecordDecl>().match(
+      FromTU, cxxRecordDecl(hasName("X")));
+  auto *FromFriend = FirstDeclMatcher<FriendDecl>().match(FromTU, friendDecl());
+  auto FromFriends = FromClass->friends();
+  unsigned int FrN = 0;
+  for (auto Fr : FromFriends) {
+    ASSERT_EQ(Fr, FromFriend);
+    ++FrN;
+  }
+  ASSERT_EQ(FrN, 1u);
+
+  Import(FromFriendF, Lang_CXX);
+  TranslationUnitDecl *ToTU = ToAST->getASTContext().getTranslationUnitDecl();
+  auto *ToClass = FirstDeclMatcher<CXXRecordDecl>().match(
+      ToTU, cxxRecordDecl(hasName("X")));
+  auto *ToFriend = FirstDeclMatcher<FriendDecl>().match(ToTU, friendDecl());
+  auto ToFriends = ToClass->friends();
+  FrN = 0;
+  for (auto Fr : ToFriends) {
+    EXPECT_EQ(Fr, ToFriend);
+    ++FrN;
+  }
+  EXPECT_EQ(FrN, 1u);
+}
+
 AST_MATCHER_P(TagDecl, hasTypedefForAnonDecl, Matcher<TypedefNameDecl>,
               InnerMatcher) {
   if (auto *Typedef = Node.getTypedefNameForAnonDecl())