diff --git a/include/stdex/compat.hpp b/include/stdex/compat.hpp
index c73266921..961555d69 100644
--- a/include/stdex/compat.hpp
+++ b/include/stdex/compat.hpp
@@ -162,6 +162,12 @@
 #endif
 #endif
 
+#if _HAS_CXX17
+#define _Constexpr_ constexpr
+#else
+#define _Constexpr_
+#endif
+
 #ifdef _MSC_VER
 #define _Deprecated_(message) __declspec(deprecated(message))
 #else
diff --git a/include/stdex/unicode.hpp b/include/stdex/unicode.hpp
index 369592fb9..31f2ec58e 100644
--- a/include/stdex/unicode.hpp
+++ b/include/stdex/unicode.hpp
@@ -101,7 +101,7 @@ namespace stdex
 				return;
 			}
 
-			if constexpr (sizeof(T_from) == sizeof(char) && sizeof(T_to) == sizeof(wchar_t)) {
+			if _Constexpr_ (sizeof(T_from) == sizeof(char) && sizeof(T_to) == sizeof(wchar_t)) {
 				assert(count_src < INT_MAX || count_src == SIZE_MAX);
 
 				// Try to convert to stack buffer first.
@@ -124,7 +124,7 @@ namespace stdex
 				throw std::runtime_error("MultiByteToWideChar failed");
 			}
 
-			if constexpr (sizeof(T_from) == sizeof(wchar_t) && sizeof(T_to) == sizeof(char)) {
+			if _Constexpr_ (sizeof(T_from) == sizeof(wchar_t) && sizeof(T_to) == sizeof(char)) {
 				assert(count_src < INT_MAX || count_src == SIZE_MAX);
 
 				// Try to convert to stack buffer first.
@@ -147,7 +147,7 @@ namespace stdex
 				throw std::runtime_error("WideCharToMultiByte failed");
 			}
 
-			if constexpr (sizeof(T_from) == sizeof(char) && sizeof(T_to) == sizeof(char)) {
+			if _Constexpr_ (sizeof(T_from) == sizeof(char) && sizeof(T_to) == sizeof(char)) {
 				assert(count_src < INT_MAX || count_src == SIZE_MAX);
 
 				// Try to convert to stack buffer first.