From 1bf360c037aed5fc854cf1fcacaa65894422e0f0 Mon Sep 17 00:00:00 2001
From: Nick Krichevsky <nick@ollien.com>
Date: Tue, 5 Oct 2021 10:56:58 -0400
Subject: [PATCH] Include <optional> if possible to check if we can use the
 feature

Addresses #303
---
 include/cxxopts.hpp | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/cxxopts.hpp b/include/cxxopts.hpp
index af87540..8fe2646 100644
--- a/include/cxxopts.hpp
+++ b/include/cxxopts.hpp
@@ -51,9 +51,14 @@ THE SOFTWARE.
 #  include <regex>
 #endif  // CXXOPTS_NO_REGEX
 
-#ifdef __cpp_lib_optional
-#include <optional>
-#define CXXOPTS_HAS_OPTIONAL
+// Nonstandard before C++17, which is coincidentally what we also need for <optional>
+#ifdef __has_include
+#  if __has_include(<optional>)
+#    include <optional>
+#    ifdef __cpp_lib_optional
+#      define CXXOPTS_HAS_OPTIONAL
+#    endif
+#  endif
 #endif
 
 #if __cplusplus >= 201603L
@@ -1355,7 +1360,7 @@ namespace cxxopts
     {
       return m_count;
     }
-    
+
 #if defined(CXXOPTS_NULL_DEREF_IGNORE)
 #pragma GCC diagnostic pop
 #endif