Why does gcc allow a const object without a user-declared default constructor but not clang?

Question

Recently Why does a const object requires a user-provided default constructor? was marked a duplicate of Why does C++ require a user-provided default constructor to default-

Answer 1

The spec currently requires user-provided default constructors but it appears that GCC is implementing a change based on DR 253 which says that if all sub-objects would be initialized without a user provided default constructor then a user-provided default constructor is not required.

This change is only draft status, has not been accepted yet and is not part of the standard. So I think think this is behavior intended by GCC developers but I'm not sure if this is a conforming extension though.

Here's a change to the first example which causes GCC to produce an error:

class A {
public:
    void f() {}

    int i;
};

int main()
{
    A a;       // OK
    const A b; // ERROR

    a.f();
    return 0;
}

Note that gcc downgrades the error to a warning with the -fpermissive flag.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42844