What is the purpose of the sentence "Absent default member initializers, ..." in the Note in [class.union]/3?

Question

[class.union]/3

A union can have member functions (including constructors and destructors), but it shall not have virtual ([class.virtual]) functions. A union shall not have base classes. A union shall not be used as a base class. If a union contains a non-static data member of reference type the program is ill-formed. [?Note: Absent default member initializers ([class.mem]), if any non-static data member of a union has a non-trivial default constructor ([class.default.ctor]), copy constructor, move constructor ([class.copy.ctor]), copy assignment operator, move assignment operator ([class.copy.assign]), or destructor ([class.dtor]), the corresponding member function of the union must be user-provided or it will be implicitly deleted ([dcl.fct.def.delete]) for the union. —?end note?]

In the code below clang emits the same error message irrespective of whether the data member U::i has a default member initializer or not. See demo.

#include <iostream>

struct S{
    int j = 1;
    S(const S&) {};
};

union U
{
    int i = 1;
    S s;
};

int main()
{
    U u;
}

Error message and note emitted by the compiler:

error: call to implicitly-deleted default constructor of 'U'  
U u;

note: default constructor of 'U' is implicitly deleted because field 's' has no default constructor
S s;

Answer 1

You're parsing the sentence incorrectly. The Absent default member initializers clause is stating that a union member with a non-trivial default constructor can be initialized by a default member initializer instead of having to provide a default constructor for the union.

It's not saying that providing a default member initializer for a different member is sufficient to avoid the constructor requirement, which seems to be how you've read it.

The following example is valid because of the clause you highlight

struct S{
    int j = 1;
    S(int) {};
};

union U
{
    int i;
    S s = 1;
};

int main()
{
    U u;
}