From b07383ead13d46ead9090fcd80dbd79ae6154bac Mon Sep 17 00:00:00 2001
From: Rian Quinn <rianquinn@gmail.com>
Date: Tue, 18 Oct 2016 12:52:45 -0600
Subject: [PATCH] [gsl_util] Update narrow_cast to use std::forward

* Update narrow_cast to use std::forward

Based on [F19](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#f19-for-forward-parameters-pass-by-tp-and-only-stdforward-the-parameter), I believe `gsl::narrow_cast` should be implemented using forward semantics.

* Fix for VS 2013
---
 gsl/gsl_util | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/gsl/gsl_util b/gsl/gsl_util
index 17a1edb..f0ac964 100644
--- a/gsl/gsl_util
+++ b/gsl/gsl_util
@@ -93,11 +93,19 @@ inline final_act<F> finally(F&& f) noexcept
 }
 
 // narrow_cast(): a searchable way to do narrowing casts of values
+#if _MSC_VER <= 1800
 template <class T, class U>
 inline constexpr T narrow_cast(U u) noexcept
 {
     return static_cast<T>(u);
 }
+#else
+template <class T, class U>
+inline constexpr T narrow_cast(U&& u) noexcept
+{
+    return static_cast<T>(std::forward<U>(u));
+}
+#endif
 
 struct narrowing_error : public std::exception
 {