Throwing exception from member constructor (brace initializer vs initializer list)

Question

I have lost faith (and probably 2 hours) in my understanding of what happens when exception is thrown (from constructor) during initialization of member object.

Let me show you the example:

int init (int f) {
    throw f;
}

struct X {
    X (int f) : n {init (f)} {}
    int n;
};

struct P {
    X x {20};
};

and the usage:

int main (int argc, char** argv) {
    try {
        P p {};
    }
    catch (int n) {
        std::cout << n << "\n";
    }
}

This code (C++11 mode) compiles fine (using GCC 7.2.1) and under the Linux (Centos 7.4.1708) I get:

terminate called after throwing an instance of 'int'
[1]    1242 abort (core dumped)  ./main

Question is: Why? Why catch() section was not taken?

I have tracked the issue, meaning that when my P class looks a little bit different:

struct P {
    P (int f) : x {f} {}
    X x;
};

and with initialization of p object in this form: P p {20} result is: 20, which was expected, no core dump.

Could someone explain me what is the difference between initializing a member using brace-or-equal-initializer and member-initializer-list in the context of exceptions?

Bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85363

Answer 1

There should be no practical difference between:

struct P {
    X x {20};
};

and

struct P {
    P() : x{20} { }
    X x;
};

But there's a GCC bug for C++11 mode (but not C++14 or later) which causes the initialization of p{} to happen outside the try-block, or adds the equivalent of noexcept to the implicitly-defined default constructor for P.