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

1 files changed, 18 insertions, 0 deletions

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));
 }