Fixed error C2766 with flag /Zc:__cplusplus on VS2017+ (fixes #1250)

CHANGELOG.md

@@ -6,6 +6,7 @@ HEAD
 
 * CMake: don't build tests when imported in another project
 * CMake: made project arch-independent
+* Visual Studio: fixed error C2766 with flag `/Zc:__cplusplus` (issue #1250)
 
 v6.15.1 (2020-04-08)
 -------

extras/tests/CMakeLists.txt

@@ -70,6 +70,12 @@ if(MSVC)
 		/W4 # Set warning level
 		/WX # Treats all compiler warnings as errors.
 	)
+
+	if (NOT MSVC_VERSION LESS  1910) #  >= Visual Studio 2017
+		add_compile_options(
+			/Zc:__cplusplus  # Enable updated __cplusplus macro
+		)
+	endif()
 endif()
 
 include_directories(Helpers)

src/ArduinoJson/Configuration.hpp

@@ -4,12 +4,6 @@
 
 #pragma once
 
-#if defined(_MSC_VER)
-#define ARDUINOJSON_HAS_INT64 1
-#else
-#define ARDUINOJSON_HAS_INT64 0
-#endif
-
 #if __cplusplus >= 201103L
 #define ARDUINOJSON_HAS_LONG_LONG 1
 #define ARDUINOJSON_HAS_NULLPTR 1
@@ -20,6 +14,12 @@
 #define ARDUINOJSON_HAS_RVALUE_REFERENCES 0
 #endif
 
+#if defined(_MSC_VER) && !ARDUINOJSON_HAS_LONG_LONG
+#define ARDUINOJSON_HAS_INT64 1
+#else
+#define ARDUINOJSON_HAS_INT64 0
+#endif
+
 // Small or big machine?
 #ifndef ARDUINOJSON_EMBEDDED_MODE
 #if defined(ARDUINO)                /* Arduino*/                 \