1 files changed, 22 insertions, 0 deletions
diff --git a/src/libutil/concepts.hh b/src/libutil/concepts.hh
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index 000000000..48bd1dbe1
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+++ b/src/libutil/concepts.hh
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+#pragma once
+/// @file Defines C++ 20 concepts that std doesn't have.
+
+#include <type_traits>
+
+namespace nix
+{
+
+/// Like std::invocable<>, but also constrains the return type as well.
+///
+/// Somehow, there is no std concept to do this, even though there is a type trait
+/// for it.
+///
+/// @tparam CallableT The type you want to constrain to be callable, and to return
+/// @p ReturnT when called with @p Args as arguments.
+///
+/// @tparam ReturnT The type the callable should return when called.
+/// @tparam Args The arguments the callable should accept to return @p ReturnT.
+template<typename CallableT, typename ReturnT, typename ...Args>
+concept InvocableR = std::is_invocable_r_v<ReturnT, CallableT, Args...>;
+
+}