Add a static_cast in args_t::size to silence a conversion warning (#966)

* Add a static_cast in `args_t::size` to silence a conversion warning

Signed-off-by: Neil Henderson <2060747+bluetarpmedia@users.noreply.github.com>

* Change `static_cast` to `unsafe_narrow`

I know, but negative numbers...

---------

Signed-off-by: Neil Henderson <2060747+bluetarpmedia@users.noreply.github.com>
Co-authored-by: Herb Sutter <herb.sutter@gmail.com>

Author: bluetarpmedia

include/cpp2util.h

@@ -1727,6 +1727,19 @@ class finally_presuccess
 };
 
 
+//-----------------------------------------------------------------------
+//
+//  An implementation of GSL's narrow_cast with a clearly 'unsafe' name
+//
+//-----------------------------------------------------------------------
+//
+template <typename C, typename X>
+constexpr auto unsafe_narrow( X&& x ) noexcept -> decltype(auto)
+{
+    return static_cast<C>(CPP2_FORWARD(x));
+}
+
+
 //-----------------------------------------------------------------------
 //
 //  args: see main() arguments as a container of string_views
@@ -1771,7 +1784,7 @@ struct args_t
     auto end()    const -> iterator    { return iterator{ argc, argv, argc }; }
     auto cbegin() const -> iterator    { return begin(); }
     auto cend()   const -> iterator    { return end(); }
-    auto size()   const -> std::size_t { return argc; }
+    auto size()   const -> std::size_t { return cpp2::unsafe_narrow<std::size_t>(argc); }
     auto ssize()  const -> int         { return argc; }
 
     auto operator[](int i) const {
@@ -1800,19 +1813,6 @@ template<typename T>
 using alien_memory = T volatile;
 
 
-//-----------------------------------------------------------------------
-//
-//  An implementation of GSL's narrow_cast with a clearly 'unsafe' name
-//
-//-----------------------------------------------------------------------
-//
-template <typename C, typename X>
-constexpr auto unsafe_narrow( X&& x ) noexcept -> decltype(auto)
-{
-    return static_cast<C>(CPP2_FORWARD(x));
-}
-
-
 //-----------------------------------------------------------------------
 //
 //  has_flags:  query whether a flag_enum value has all flags in 'flags' set