In the latest draft of the C++ standard, [basic.life]/1 states:

The lifetime of an object or reference is a runtime property of the object or reference. A variable is said to have vacuous initialization if it is default-initialized and, if it is of class type or a (possibly multi-dimensional) array thereof, that class type has a trivial default constructor. The lifetime of an object of type T begins when:

• storage with the proper alignment and size for type T is obtained, and
• its initialization (if any) is complete (including vacuous initialization) ([dcl.init]),

except that if the object is a union member or subobject thereof, its lifetime only begins if that union member is the initialized member in the union ([dcl.init.aggr], [class.base.init]), or as described in [class.union]. [...]

From that paragraph I understand that the only way a member of a union begins its lifetime is:

• if that member "is the initialized member in the union" (e.g. if it is referenced in a mem-initializer), or
• some other way mentioned in [class.union]

However, the only normative paragraph in [class.union] that specifies how a union member can begin its lifetime is [class.union]/5 (but it only applies to specific types, i.e. either non-class, non-array, or class type with a trivial constructor that is not deleted, or array of such types).

The next paragraph, [class.union]/6 (comprising a note and an example, therefore it contains no normative text), describes a way to change the active member of a union, by using a placement new-expression, such as new (&u.n) N. However, based on [basic.life]/1, the lifetime of u.n should not start when using the placement new-expression.

If I am not missing anything, I believe that either [basic.life]/1 should be changed or a new paragraph must be added to [class.union].