[P0870R6] A proposal for a type trait to detect narrowing conversions

[saki7]

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p0870r6.html

template<class From, class To>
concept is_convertible_without_narrowing = std::is_convertible_v<From, To> && 
  requires (From&& x) {
    { std::type_identity_t<To[]>{std::forward<From>(x)} } -> std::same_as<To[1]>;
  };

I think our version (quoted below) can be rewritten with the standard utility above, but we must make sure that our test suite passes, and also that the IDE completion will not fail

iris/include/iris/type_traits.hpp

Lines 240 to 256 in ef4da2a

	// This version works better than MSVC's, does not break IntelliSense or ReSharper
	template<std::size_t I, class Ti>
	struct aggregate_initialize_overload
	{
	using TiA = Ti[];
	// https://eel.is/c++draft/dcl.init.general#14
	// https://eel.is/c++draft/dcl.init.list#3.4
	// https://eel.is/c++draft/dcl.init.aggr#3
	template<class T>
	auto operator()(Ti, T&&) -> aggregate_initialize_tag<I, Ti>
	requires requires(T&& t) { { TiA{std::forward<T>(t)} }; } // emulate `Ti x[] = {std::forward<T>(t)};`
	{
	return {}; // silence MSVC warning
	}
	};

[saki7]

This utility is also used in existing X4 code, where it currently uses some ad-hoc implementation