Fix "Undefined symbols" linker error when DocumentChange::npos was used (#474)

Author: dconeybe

firestore/integration_test/src/integration_test.cc

@@ -651,6 +651,14 @@ TEST_F(FirebaseFirestoreBasicTest, TestQuery) {
                   "int", firebase::firestore::FieldValue::Integer(101))))));
 }
 
+TEST_F(FirebaseFirestoreBasicTest, TestDocumentChangeNpos) {
+  // This test may seem pointless, but it exists to avoid the long-standing
+  // latent bug that `npos` was not defined on non-Android platforms and
+  // would therefore fail to link if used.
+  EXPECT_EQ(firebase::firestore::DocumentChange::npos,
+            static_cast<std::size_t>(-1));
+}
+
 TEST_F(FirebaseFirestoreBasicTest,
        TestInvalidatingReferencesWhenDeletingFirestore) {
   delete firestore_;

firestore/integration_test_internal/src/integration_test.cc

@@ -651,6 +651,14 @@ TEST_F(FirebaseFirestoreBasicTest, TestQuery) {
                   "int", firebase::firestore::FieldValue::Integer(101))))));
 }
 
+TEST_F(FirebaseFirestoreBasicTest, TestDocumentChangeNpos) {
+  // This test may seem pointless, but it exists to avoid the long-standing
+  // latent bug that `npos` was not defined on non-Android platforms and
+  // would therefore fail to link if used.
+  EXPECT_EQ(firebase::firestore::DocumentChange::npos,
+            static_cast<std::size_t>(-1));
+}
+
 TEST_F(FirebaseFirestoreBasicTest,
        TestInvalidatingReferencesWhenDeletingFirestore) {
   delete firestore_;

firestore/src/common/document_change.cc

@@ -23,6 +23,8 @@ using Type = DocumentChange::Type;
 // Older NDK (r16b) fails to define this properly. Fix this when support for
 // the older NDK is removed.
 const std::size_t DocumentChange::npos = static_cast<std::size_t>(-1);
+#else
+constexpr std::size_t DocumentChange::npos;
 #endif  // defined(ANDROID)
 
 DocumentChange::DocumentChange() {}

release_build_files/readme.md

@@ -567,6 +567,11 @@ code.
 
 ## Release Notes
 
+### 8.1.0
+-   Changes
+    -   Firestore: Fixed a linker error when `DocumentChange::npos` was used.
+        ([#474](https://github.com/firebase/firebase-cpp-sdk/pull/474)).
+
 ### 8.0.0
 -   Changes
     -   Analytics: Removed `SetCurrentScreen()` following its removal from iOS SDK