libutil: add concepts for constraining iterables ("ranges")

Change-Id: I5d75abb9d3713a3fab863666786aa8d38cc93dbd

lix/libutil/concepts.hh

@@ -1,6 +1,7 @@
 #pragma once
 /// @file Defines C++ 20 concepts that std doesn't have.
 
+#include <ranges>
 #include <type_traits>
 
 namespace nix
@@ -19,4 +20,25 @@ namespace nix
 template<typename CallableT, typename ReturnT, typename ...Args>
 concept InvocableR = std::is_invocable_r_v<ReturnT, CallableT, Args...>;
 
+/// @brief Like std::ranges::viewable_range<>, but also verifies the range's value type.
+///
+/// Because C++ sucks ass, the check you want against the range's value type
+/// might vary wildly. So this concept takes a *template template* argument,
+/// which should be a type-trait like template-struct, which takes two template type
+/// arguments, the first being std::ranges::range_value_t<RangeT>, and the other being
+/// ElemT.
+///
+/// @tparam RangeT The range type you want to constrain.
+/// @tparam ItemT The item type you want RangeT's item type to be "like".
+/// @tparam LikeT The to check if RangeT's item type is "like" ItemT.
+template<typename RangeT, typename ItemT, template<typename, typename> typename LikeT>
+concept ViewOfLike =
+    std::ranges::viewable_range<RangeT> && LikeT<std::ranges::range_value_t<RangeT>, ItemT>::value;
+
+/// @brief Application of @ref ViewOfLike using @ref std::is_nothrow_convertible.
+///
+/// @tparam RangeT The range type to constrain.
+/// @tparam ItemT The item type you want RangeT's item type to be nothrow-convertible to.
+template<typename RangeT, typename ItemT>
+concept ViewOf = ViewOfLike<RangeT, ItemT, std::is_nothrow_convertible>;
 }