Rationale (August, 2011):

As given in the preceding note.

Note (March, 2008):

This issue was resolved by the adoption of paper J16/08-0054 = WG21 N2544 (“Unrestricted Unions”) at the Bellevue meeting.

Can a member of a union be of a class that has a user-declared non-default constructor? The restrictions on union membership in 11.5 [class.union] paragraph 1 only mention default and copy constructors:

An object of a class with a non-trivial default constructor (11.4.5 [class.ctor]), a non-trivial copy constructor (11.4.5.3 [class.copy.ctor]), a non-trivial destructor (11.4.7 [class.dtor]), or a non-trivial copy assignment operator (12.4.3.2 [over.ass], 11.4.5.3 [class.copy.ctor]) cannot be a member of a union...

(11.4.5 [class.ctor] paragraph 11 does say, “a non-trivial constructor,” but it's not clear whether that was intended to refer only to default and copy constructors or to any user-declared constructor. For example, 6.7.7 [class.temporary] paragraph 3 also speaks of a “non-trivial constructor,” but the cross-references there make it clear that only default and copy constructors are in view.)