From 8a4b9ed0bf643726ce625678a17b1fc40d90870c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Johel=20Ernesto=20Guerrero=20Pe=C3=B1a?=
 <johelegp@gmail.com>
Date: Wed, 23 Jun 2021 18:28:45 -0400
Subject: [PATCH] feat: narrow for non totally ordered types (#986)

---
 include/gsl/narrow    | 19 ++++++++++++++++++-
 tests/utils_tests.cpp |  5 +++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/gsl/narrow b/include/gsl/narrow
index 67aac05..40016d1 100644
--- a/include/gsl/narrow
+++ b/include/gsl/narrow
@@ -26,7 +26,7 @@ struct narrowing_error : public std::exception
 };
 
 // narrow() : a checked version of narrow_cast() that throws if the cast changed the value
-template <class T, class U>
+template <class T, class U, typename std::enable_if<std::is_arithmetic<T>::value>::type* = nullptr>
 // clang-format off
 GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
 GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recognise noexcept(false)
@@ -45,5 +45,22 @@ GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recogn
 
     return t;
 }
+
+template <class T, class U, typename std::enable_if<!std::is_arithmetic<T>::value>::type* = nullptr>
+// clang-format off
+GSL_SUPPRESS(type.1) // NO-FORMAT: attribute
+GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recognise noexcept(false)
+    // clang-format on
+    constexpr T narrow(U u) noexcept(false)
+{
+    const T t = narrow_cast<T>(u);
+
+    if (static_cast<U>(t) != u)
+    {
+        throw narrowing_error{};
+    }
+
+    return t;
+}
 } // namespace gsl
 #endif // GSL_NARROW_H
diff --git a/tests/utils_tests.cpp b/tests/utils_tests.cpp
index 0585c79..39b4ca2 100644
--- a/tests/utils_tests.cpp
+++ b/tests/utils_tests.cpp
@@ -17,6 +17,7 @@
 #include <gtest/gtest.h>
 
 #include <algorithm>   // for move
+#include <complex>
 #include <cstddef>     // for std::ptrdiff_t
 #include <functional>  // for reference_wrapper, _Bind_helper<>::type
 #include <gsl/narrow>  // for narrow, narrowing_error
@@ -144,5 +145,9 @@ TEST(utils_tests, narrow)
 
     n = -42;
     EXPECT_THROW(narrow<unsigned>(n), narrowing_error);
+
+    EXPECT_TRUE(
+        narrow<std::complex<float>>(std::complex<double>(4, 2)) == std::complex<float>(4, 2));
+    EXPECT_THROW(narrow<std::complex<float>>(std::complex<double>(4.2)), narrowing_error);
 }
 #endif // GSL_KERNEL_MODE