Force C++17 for MSVC to fix Firestore linkage errors

Author: google-labs-jules[bot]

CMakeLists.txt

@@ -19,6 +19,10 @@ cmake_minimum_required (VERSION 3.1)
 set (CMAKE_CXX_STANDARD 17)
 set (CMAKE_CXX_STANDARD_REQUIRED YES)  # Don't fall back to an earlier version.
 
+if(MSVC)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /std:c++17")
+endif()
+
 # Turn on virtual folders for visual studio
 set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 

STYLE_GUIDE.md

@@ -100,10 +100,10 @@ Examples of typical Do's and Don'ts:
   they result in hard to understand errors, and can make correct use of
   your API harder to understand.
 
-* Utilize C++11 features where appropriate. This project has adopted C++11,
+* Utilize C++17 features where appropriate. This project has adopted C++17,
   and features such as `std::unique_ptr`, `std::shared_ptr`,
-  `std::make_unique`, and `std::move` are encouraged to improve code safety
-  and readability. However, avoid features from C++14 or newer standards.
+  `std::make_unique`, `std::move`, and `std::optional` are encouraged to
+  improve code safety and readability.
 
 * Go easy on objectifying everything, and prefer value types. In languages
   like Java it is common to give each "concept" your API deals with its own

release_build_files/readme.md

@@ -72,7 +72,7 @@ libs/windows              | Windows static libraries (desktop or stub
 
 ## C++ Language Standards
 
-The Firebase C++ SDK supports the C++14 language standard. For more information,
+The Firebase C++ SDK supports the C++17 language standard. For more information,
 please see our [C++ Language Standard Support
 Criteria](https://opensource.google/documentation/policies/cplusplus-support#c_language_standard).
 
@@ -677,6 +677,7 @@ code.
 ## Release Notes
 ### Upcoming Release
 -   Changes
+    - General: Updated the minimum C++ standard from C++14 to C++17.
     - iOS: Added an option to explicitly specify your app's `AppDelegate` class
       name via the `FirebaseAppDelegateClassName` key in `Info.plist`. This
       provides a more direct way for Firebase to interact with your specified