Define a new type in a function return value

Question

I was surprised to discover that the following code compiles, runs, AND produces the expected output under MSVC:

#include <iostream>
using namespace std;

struct Foo{
    int _x;
    Foo(int x): _x(x) {}
}  //Note: no semi-colon after class definition.
   //Makes this behave as a return type for the following function:

Foo_factory(int x)
{return Foo(x);}

int main (int argc, char* argv[])
{
    Foo foo = Foo_factory(42);
    cout << foo._x << endl;  //Prints "42"
    return 0;
}

I was less surprised to see MinGW fail to compile with the error "new types may not be defined in a return type". Is this just another Microsoft exception to the standard, or is this legal C++?

Answer 1

In N3797 (C++14) and N3485 (C++11), §8.3.5 [dcl.fct]/9 explicitly starts with:

Types shall not be defined in return or parameter types.

Thus, your code is invalid and GCC is correct to diagnose it.