Fix Firestore compilation under Xcode < 9.2 (#1367)

* Don't rely on specialization failure to determine when std::hash is unavailable.

Instead manually declare the conditions under which std::hash should be
defined.

* Fix detection of Objective-C classes in Xcode < 9.2

std::is_base_of<NSObject, NSString>{} is false there so the overloads
defined for Objective-C types weren't getting enabled.

* Add explicit tests for StringFormat using Objective-C objects

* Add explicit tests for HasStdHash

4 files changed, 137 insertions, 0 deletions

diff --git a/Firestore/core/test/firebase/firestore/util/CMakeLists.txt b/Firestore/core/test/firebase/firestore/util/CMakeLists.txt
index bcb1c84..c07a4ea 100644
--- a/Firestore/core/test/firebase/firestore/util/CMakeLists.txt
+++ b/Firestore/core/test/firebase/firestore/util/CMakeLists.txt
@@ -137,3 +137,12 @@ cc_test(
     firebase_firestore_util
     gmock
 )
+
+if(APPLE)
+  target_sources(
+    firebase_firestore_util_test
+    PUBLIC
+      string_format_apple_test.mm
+      type_traits_apple_test.mm
+  )
+endif()
diff --git a/Firestore/core/test/firebase/firestore/util/hashing_test.cc b/Firestore/core/test/firebase/firestore/util/hashing_test.cc
index e5d9ff8..2c5c2f7 100644
--- a/Firestore/core/test/firebase/firestore/util/hashing_test.cc
+++ b/Firestore/core/test/firebase/firestore/util/hashing_test.cc
@@ -16,6 +16,9 @@
 
 #include "Firestore/core/src/firebase/firestore/util/hashing.h"
 
+#include <map>
+#include <string>
+
 #include "absl/strings/string_view.h"
 #include "gtest/gtest.h"
 
@@ -29,6 +32,21 @@ struct HasHashMember {
   }
 };
 
+TEST(HashingTest, HasStdHash) {
+  EXPECT_TRUE(impl::has_std_hash<float>::value);
+  EXPECT_TRUE(impl::has_std_hash<double>::value);
+  EXPECT_TRUE(impl::has_std_hash<int>::value);
+  EXPECT_TRUE(impl::has_std_hash<int64_t>::value);
+  EXPECT_TRUE(impl::has_std_hash<std::string>::value);
+  EXPECT_TRUE(impl::has_std_hash<void*>::value);
+  EXPECT_TRUE(impl::has_std_hash<const char*>::value);
+
+  struct Foo {};
+  EXPECT_FALSE(impl::has_std_hash<Foo>::value);
+  EXPECT_FALSE(impl::has_std_hash<absl::string_view>::value);
+  EXPECT_FALSE((impl::has_std_hash<std::map<std::string, std::string>>::value));
+}
+
 TEST(HashingTest, Int) {
   ASSERT_EQ(std::hash<int>{}(0), Hash(0));
 }
diff --git a/Firestore/core/test/firebase/firestore/util/string_format_apple_test.mm b/Firestore/core/test/firebase/firestore/util/string_format_apple_test.mm
new file mode 100644
index 0000000..f0bcd35
--- /dev/null
+++ b/Firestore/core/test/firebase/firestore/util/string_format_apple_test.mm
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "Firestore/core/src/firebase/firestore/util/string_format.h"
+
+#import <Foundation/NSString.h>
+
+#include "gtest/gtest.h"
+
+@interface FSTDescribable : NSObject
+@end
+
+@implementation FSTDescribable
+
+- (NSString*)description {
+  return @"description";
+}
+
+@end
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+TEST(StringFormatTest, NSString) {
+  EXPECT_EQ("Hello World", StringFormat("Hello %s", @"World"));
+
+  NSString* hello = [NSString stringWithUTF8String:"Hello"];
+  EXPECT_EQ("Hello World", StringFormat("%s World", hello));
+
+  // NOLINTNEXTLINE false positive on "string"
+  NSMutableString* world = [NSMutableString string];
+  [world appendString:@"World"];
+  EXPECT_EQ("Hello World", StringFormat("Hello %s", world));
+}
+
+TEST(StringFormatTest, FSTDescribable) {
+  FSTDescribable* desc = [[FSTDescribable alloc] init];
+  EXPECT_EQ("Hello description", StringFormat("Hello %s", desc));
+
+  id desc_id = desc;
+  EXPECT_EQ("Hello description", StringFormat("Hello %s", desc_id));
+}
+
+}  //  namespace util
+}  //  namespace firestore
+}  //  namespace firebase
diff --git a/Firestore/core/test/firebase/firestore/util/type_traits_apple_test.mm b/Firestore/core/test/firebase/firestore/util/type_traits_apple_test.mm
new file mode 100644
index 0000000..dfb03bb
--- /dev/null
+++ b/Firestore/core/test/firebase/firestore/util/type_traits_apple_test.mm
@@ -0,0 +1,50 @@
+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "Firestore/core/src/firebase/firestore/util/type_traits.h"
+
+#import <Foundation/NSArray.h>
+#import <Foundation/NSObject.h>
+#import <Foundation/NSString.h>
+
+#include "gtest/gtest.h"
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+TEST(TypeTraitsTest, IsObjectiveCPointer) {
+  static_assert(is_objective_c_pointer<NSObject*>{}, "NSObject");
+  static_assert(is_objective_c_pointer<NSString*>{}, "NSString");
+  static_assert(is_objective_c_pointer<NSArray<NSString*>*>{},
+                "NSArray<NSString*>");
+
+  static_assert(is_objective_c_pointer<id>{}, "id");
+  static_assert(is_objective_c_pointer<id<NSCopying>>{}, "id<NSCopying>");
+
+  static_assert(!is_objective_c_pointer<int*>{}, "int*");
+  static_assert(!is_objective_c_pointer<void*>{}, "void*");
+  static_assert(!is_objective_c_pointer<int>{}, "int");
+  static_assert(!is_objective_c_pointer<void>{}, "void");
+
+  struct Foo {};
+  static_assert(!is_objective_c_pointer<Foo>{}, "Foo");
+  static_assert(!is_objective_c_pointer<Foo*>{}, "Foo");
+}
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase