The standard dictates that decltype(*r) deduces to T&, not T. The std::decay is there to collapse the reference type into a value type. Therefore your simplific

The standard dictates that decltype(*r) deduces to T&, not T. The std::decay is there to collapse the reference type into a value type. Therefore your simplification results in a compilation error. An alternative way to get the value type is to use std::remove_pointer_t: using ResourceType = std::remove_pointer_t<decltype(c(args...))>;