"static functions with block scope are illegal" error depending on initialisation style?

Question

I have a class Library containing a struct Transaction, and the struct has a member variable of type Patron.

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

class Library {
public:
    struct Transaction {
        Patron p;
        Transaction(Patron pp) :p(pp) { }
        Transaction();
    };
};

For the default constructor of Transaction, I have a function default_transaction() returning a const reference to a static object as recommended by Stroustrup in "Programming – Principles and Practices using C++" (p. 324); reasoning: avoid building default value into constructor code, avoid global variable and associated initialisation problems.

const Library::Transaction& default_transaction()
{
    static Library::Transaction tt = Library::Transaction(Patron());
    return tt;
}

Library::Transaction::Transaction()
    :p(default_transaction().p) { }

int main()
{
    Library::Transaction t;
}

If I change this line

static Library::Transaction tt = Library::Transaction(Patron());

to

static Library::Transaction tt(Patron());

i.e., from "explicit" initialisation style to "colloquial", I get an error: "static functions with block scope are illegal".

Why is that? Aren't the two equivalent? And what does the error mean exactly?

Answer 1

static Library::Transaction tt(Patron());

This is parsed as the declaration of a static (storage, not static member) function named tt that returns a Library::Transaction and takes a unnamed function as single argument. The argument function takes no values and returns a Patron.