From de7027a40383b67ea3a52e1475fde097d39f7c70 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 13 Mar 2019 17:38:01 +0200
Subject: Fix std::optional usage test for VC 16.0

---
 libbutl/optional.mxx | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/libbutl/optional.mxx b/libbutl/optional.mxx
index f41aab4..601e054 100644
--- a/libbutl/optional.mxx
+++ b/libbutl/optional.mxx
@@ -19,10 +19,13 @@
 #elif defined(_MSC_VER)
    //
    // Available from 19.10 (15.0). Except it (or the compiler) doesn't seem to
-   // be constexpr-correct. Let's re-test with 16.0.
+   // be constexpr-correct. Things appear to be fixed in 19.20 (16.0) but
+   // optional is now only available in the C++17 mode or later.
    //
-#  if _MSC_VER > 1916
-#    define LIBBUTL_STD_OPTIONAL
+#  if _MSC_VER >= 1920
+#    if defined(_MSVC_LANG) && _MSVC_LANG >= 201703L // See /Zc:__cplusplus
+#      define LIBBUTL_STD_OPTIONAL
+#    endif
 #  endif
 #elif defined(__clang__)
    //
-- 
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