SERVER-38520 Fix std::variant feature detection

1 files changed, 10 insertions, 4 deletions

diff --git a/src/mongo/stdx/variant.h b/src/mongo/stdx/variant.h
index 374432fac5e..4de52b0fe2d 100644
--- a/src/mongo/stdx/variant.h
+++ b/src/mongo/stdx/variant.h
@@ -30,15 +30,21 @@
 
 #pragma once
 
-#ifndef __cpp_lib_variant
-#include "third_party/variant-1.3.0/include/mpark/variant.hpp"
-#else
+// Feature detection is messy in C++17 (to be improved in C++20 with the <version> header).
+// __cpp_lib_variant is available to check after <variant> is included. Include it if it's available
+// (but only if C++17 as MSVS makes it available but emits a message if used). If the variant lib is
+// not high enough version, fall back to mpark.
+#if __cplusplus >= 201703L && defined(__has_include) && __has_include(<variant>)
 #include <variant>
 #endif
 
+#if !defined(__cpp_lib_variant) || __cpp_lib_variant < 201603
+#include "third_party/variant-1.3.0/include/mpark/variant.hpp"
+#endif
+
 namespace mongo {
 namespace stdx {
-#if __cplusplus < 201703L || !defined(__cpp_lib_variant)
+#if !defined(__cpp_lib_variant) || __cpp_lib_variant < 201603
 using ::mpark::variant;
 using ::mpark::visit;
 using ::mpark::holds_alternative;