Deduction guides are not found by name lookup.

Instead, when performing class template argument deduction ([over.match.class.deduct]), all reachable deduction guides declared for the class template are considered.

[Example 1: template<class T, class D = int> struct S { T data; }; template<class U> S(U) -> S<typename U::type>; struct A { using type = short; operator type(); }; S x{A()}; — end example]

A deduction-guide shall inhabit the scope to which the corresponding class template belongs and, for a member class template, have the same access.

Two deduction guide declarations for the same class template shall not have equivalent parameter-declaration-clauses if either is reachable from the other.