Issue #329, fixed build issues on clang

Fixed build issues that clang shows up the MS VS accepts.

gsl/gsl_byte

@@ -106,14 +106,15 @@ constexpr IntegerType to_integer(byte b) noexcept
     return {b};
 }
 
-template<bool, typename T>
+template<bool E, typename T>
 constexpr byte to_byte_impl(T t) noexcept
 {
     static_assert(
-        false, 
+        E,
         "gsl::to_byte(t) must be provided an unsigned char, otherwise data loss may occur. "
         "If you are calling to_byte with an integer contant use: gsl::to_byte<t>() version."
-    )
+    );
+    return static_cast<byte>(t);
 }
 template<>
 constexpr byte to_byte_impl<true, unsigned char>(unsigned char t) noexcept