“CPP reference collapsing – the C++ language rule for applying & or && to a type alias or decltype expression that is itself a reference (T& or T&&), i.e., when there are two reference operators being applied to the same underlying type. The resulting reference will be an lvalue reference (T&) unless both operators are &&, in which case the result will be an rvalue reference (T&&). Forwarding References (380)” (EMCppSfe 2021)