From 6eeed739f1acb81c8b69ceae11bd5f350dab3601 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Johel=20Ernesto=20Guerrero=20Pe=C3=B1a?=
 <johelegp@gmail.com>
Date: Mon, 14 Jan 2019 22:37:37 -0400
Subject: [PATCH] Make narrow constexpr (#698)

---
 include/gsl/gsl_util  | 9 +++++++++
 tests/utils_tests.cpp | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/include/gsl/gsl_util b/include/gsl/gsl_util
index 542bbaa..1558044 100644
--- a/include/gsl/gsl_util
+++ b/include/gsl/gsl_util
@@ -37,6 +37,12 @@
 #endif            // _MSC_VER < 1910
 #endif            // _MSC_VER
 
+#if (defined(_MSC_VER) && _MSC_VER < 1910) || (!defined(__clang__) && defined(__GNUC__) && __GUNC__ < 6)
+#define GSL_CONSTEXPR_NARROW 0
+#else
+#define GSL_CONSTEXPR_NARROW 1
+#endif
+
 namespace gsl
 {
 //
@@ -111,6 +117,9 @@ namespace details
 template <class T, class U>
 GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
 GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recognise noexcept(false)
+#if GSL_CONSTEXPR_NARROW
+constexpr
+#endif
 T narrow(U u) noexcept(false)
 {
     T t = narrow_cast<T>(u);
diff --git a/tests/utils_tests.cpp b/tests/utils_tests.cpp
index 05f8d0f..5cacd97 100644
--- a/tests/utils_tests.cpp
+++ b/tests/utils_tests.cpp
@@ -126,4 +126,8 @@ TEST_CASE("narrow")
 
     n = -42;
     CHECK_THROWS_AS(narrow<unsigned>(n), narrowing_error);
+
+#if GSL_CONSTEXPR_NARROW
+    static_assert(narrow<char>(120) == 120, "Fix GSL_CONSTEXPR_NARROW");
+#endif
 }