Question

class C {
public:
    C() { }
};

class B {
public:
    B(C c) { }
    B() { }
};

class A {
public:
    A(bool b) { }
    A(B b) { }
};

int main() {
    A a1 = true; // bool -> A        is allowed
    A a2 = B();  // B -> A           is allowed

    A a3 = 7;    // int -> bool -> A is allowed
    A a4 = C();  // C -> B -> A      isn't allowed
}

Why I can use two-step implicit conversion with bool but can’t use it with C?
What is the general rule describing multistep implicit conversion?

Answer 1

There is no multi-step user-defined implicit conversion.

int -> bool -> A

is allowed because the int->bool conversion isn’t user-defined.

12.3 Conversions [class.conv]

1 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 (clause 4), for
initialization (8.5), and for explicit type conversions (5.4, 5.2.9).

2 User-defined conversions are applied only where they are unambiguous
(10.2, 12.3.2). Conversions obey the access control rules (clause 11).
Access control is applied after ambiguity resolution (3.4).

3 [ Note:
See 13.3 for a discussion of the use of conversions in function calls
as well as examples below. —end note ]

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