Why do unions have a deleted default constructor if just one of its members doesn't have one?

Question

N3797::9.5/2 [class.union] says:

If any non-static data member of a union has a non-trivial default constructor (12.1), copy c

Answer 1

The relevant wording is in C++11 [class.ctor]p5 (emphasis mine):

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted (8.4). [...] A defaulted default constructor for class X is defined as deleted if:

[...]

• X is a union-like class that has a variant member with a non-trivial default constructor,

[...]

• any direct or virtual base class, or non-static data member with no brace-or-equal-initializer, has class type M (or array thereof) and either M has no default constructor or overload resolution (13.3) as applied to M's default constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or

[...]

Your class A has no default constructor, so a defaulted default constructor (whether implicit or explicit) for a class X (whether union or non-union) containing a non-static data member of type A without an initialiser leads to the default constructor for X being deleted. It has to: there's simply no way for the compiler to generate any other default constructor.

As for your follow-up question in the comments:

If instead of A not having a default constructor, it has a non-trivial default constructor, then there is a difference between using that in a union and in a non-union class, and that is also part of [class.ctor]p5: it is the first bullet point that I included, without emphasis, in my earlier quote.