[class.conv.general]

Type conversions of class objects can be specified by constructors and by conversion functions.

These conversions are called user-defined conversions and are used for implicit type conversions ([conv]), for initialization ([dcl.init]), and for explicit type conversions ([expr.type.conv], [expr.cast], [expr.static.cast]).

At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.

[Example 1: struct X { operator int(); }; struct Y { operator X(); }; Y a; int b = a; / error: no viable conversion (a.operator X().operator int() not considered) int c = X(a); / OK, a.operator X().operator int() — end example]