The check looks for perfect forwarding constructors that can hide copy or move constructors. If a non const lvalue reference is passed to the constructor, the forwarding reference parameter will be a better match than the const reference parameter of the copy constructor, so the perfect forwarding constructor will be called, which can be confusing. For detailed description of this issue see: Scott Meyers, Effective Modern C++, Item 26.

Consider the following example:

class Person {
  // C1: perfect forwarding ctor
  template<typename T>
  explicit Person(T&& n) {}

  // C2: perfect forwarding ctor with parameter default value
  template<typename T>
  explicit Person(T&& n, int x = 1) {}

  // C3: perfect forwarding ctor guarded with enable_if
  template<typename T, typename X = enable_if_t<is_special<T>,void>>
  explicit Person(T&& n) {}

  // (possibly compiler generated) copy ctor
  Person(const Person& rhs);

The check warns for constructors C1 and C2, because those can hide copy and move constructors. We suppress warnings if the copy and the move constructors are both disabled (deleted or private), because there is nothing the perfect forwarding constructor could hide in this case. We also suppress warnings for constructors like C3 that are guarded with an enable_if, assuming the programmer was aware of the possible hiding.


For deciding whether a constructor is guarded with enable_if, we consider the default values of the type parameters and the types of the constructor parameters. If any part of these types is std::enable_if or std::enable_if_t, we assume the constructor is guarded.