Incomplete type is not allowed in a class, but is allowed in a class template

Question

The following is invalid code:

struct foo {
    struct bar;
    bar x;        // error: field x has incomplete type
    struct bar{ int value{42}; };
};

int mai         


        

Answer 1

I think this example is explicitly allowed by

17.6.1.2 Member classes of class templates [temp.mem.class]

1 A member class of a class template may be defined outside the class template definition in which it is declared. [Note: The member class must be defined before its first use that requires an instantiation (17.8.1) For example,

template struct A {
  class B;
};

A::B* b1; // OK: requires A to be defined but not A::B
template class A::B { };
A::B b2; // OK: requires A::B to be defined

—end note ]

This should work fine too:

template 
struct foo_impl {
    struct bar;
    bar x;        // no problems here
};

template
struct foo_impl::bar{ int value{42}; };

using foo = foo_impl<>;

int main()
{
    return foo{}.x.value;
}