No mention of implicit conversion of a constructor with default arguments in C.46

I think this section "C.46: By default, declare single-argument constructors explicit" could be improved if there would be a Note saying that a constructor with default arguments acts like a single-argument constructor. For instance, the following constructor (without explicit) does not avoid implicit conversion, even though it is not a single-argument constructor (?), but rather acts like the one (both parameters have defaults or only the 2nd one -- it doesn't matter):

struct Foo {
    /*explicit*/ Foo(int a = 0, int b = 0) : sum{a + b} { }
    int sum;
};

void bar() {
    Foo foo = 10; // implicit conversion
}

(?) I mean, I'm not sure, maybe Foo::Foo(int, int) shown above is a book-case example of "single-argument constructor", but I think it's worth mentioning that one should not only be aware of Foo::Foo(int a) case, but Foo::Foo(int a/*,=0*/, int b = 0/*, int c = 0, and so on */) as well.

Start with the C.46 section in the C++ Core Guidelines, especially its discussion of single-argument constructors. Add a note covering constructors with default arguments and the implicit conversion shown in the issue. Done means the guidance clearly warns readers about this case without changing the surrounding rule.