Why can you define a lambda with a name in the capture-list of a lambda-expression, but not outside of it?

Question

I've read the explanation of lambda expressions in C++, and I've seen how they work, and how a lambda-expression needs to be declared (many details omitted):

[ lambda-capture_opt ] ( parameter-declaration-clause ) compound-statement.

I've seen that you can define a lambda inside another lambda definition in a way like this:

[lambda1{[](){
    return 123;
}}]{
    return lambda1();
}

This works:

cout << [lambda1{[](){
    return 123;
}}]{
    return lambda1();
}();

This one doesn't:

cout << prova{[](){
    return "hello";
}}();

Why can I define it like lambda1{...} in the second example and not in the third?

Answer 1

In the second example, [lambda1{[](){...}}]{...} you see that the identifier lambda1 occurs within brackets[] that are the capture list of the lambda. This is a context where a new identifier, lambda1, can be declared.

In the third example, the identifier prova occurs without any "special" context, in particular, not one where a new identifier can be declared. Hence, it is looked up in the usual way, and if it is not found, you have an error.