From b8c77d4041474c3ba2eb0c003c75944136821fa1 Mon Sep 17 00:00:00 2001
From: David Schneider <davschne@gmail.com>
Date: Tue, 15 Mar 2022 17:52:11 -0700
Subject: [PATCH] Make inclusion of header <optional> opt-out via macro (#7168)

* Allow suppression of including <optional> via macro

* Combine preprocessor conditions

* Make inclusion of header <optional> opt-in

* Make inclusion of <optional> opt-out via macro

* Auto-detect C++17+ when defining macro

* Enclose macro expression in parentheses

* Check value of macro, not whether it's defined

* Fix parentheses

* Don't define macro with expression using other macros

The preprocessor doesn't seem to like it.

* Fix parentheses
---
 include/flatbuffers/stl_emulation.h | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/include/flatbuffers/stl_emulation.h b/include/flatbuffers/stl_emulation.h
index 75d13b29c..967f297d6 100644
--- a/include/flatbuffers/stl_emulation.h
+++ b/include/flatbuffers/stl_emulation.h
@@ -26,16 +26,20 @@
 #include <memory>
 #include <limits>
 
-// Detect C++17 compatible compiler.
-// __cplusplus >= 201703L - a compiler has support of 'static inline' variables.
-#if defined(FLATBUFFERS_USE_STD_OPTIONAL) \
-    || (defined(__cplusplus) && __cplusplus >= 201703L) \
-    || (defined(_MSVC_LANG) &&  (_MSVC_LANG >= 201703L))
+#ifndef FLATBUFFERS_USE_STD_OPTIONAL
+  // Detect C++17 compatible compiler.
+  // __cplusplus >= 201703L - a compiler has support of 'static inline' variables.
+  #if (defined(__cplusplus) && __cplusplus >= 201703L) \
+      || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+    #define FLATBUFFERS_USE_STD_OPTIONAL 1
+  #else
+    #define FLATBUFFERS_USE_STD_OPTIONAL 0
+  #endif // (defined(__cplusplus) && __cplusplus >= 201703L) ...
+#endif // FLATBUFFERS_USE_STD_OPTIONAL
+
+#if FLATBUFFERS_USE_STD_OPTIONAL
   #include <optional>
-  #ifndef FLATBUFFERS_USE_STD_OPTIONAL
-    #define FLATBUFFERS_USE_STD_OPTIONAL
-  #endif
-#endif // defined(FLATBUFFERS_USE_STD_OPTIONAL) ...
+#endif
 
 // The __cpp_lib_span is the predefined feature macro.
 #if defined(FLATBUFFERS_USE_STD_SPAN)
@@ -128,7 +132,7 @@ namespace flatbuffers {
   };
 #endif  // defined(FLATBUFFERS_TEMPLATES_ALIASES)
 
-#ifdef FLATBUFFERS_USE_STD_OPTIONAL
+#if FLATBUFFERS_USE_STD_OPTIONAL
 template<class T>
 using Optional = std::optional<T>;
 using nullopt_t = std::nullopt_t;