[Voted into WP at October, 2009 meeting.]

Proposed resolution (July, 2009):

1. Change 11.5 [class.union] paragraph 1 as follows:

...If a union contains a non-static data member of reference type the program is ill-formed. At most one non-static data member of a union shall have a brace-or-equal-initializer. [Note:...

2. Change 11.9.3 [class.base.init] paragraph 5 as follows:

...If a ctor-initializer specifies more than one mem-initializer for the same member, or for the same base class or for multiple members of the same union (including members of anonymous unions), the ctor-initializer is ill-formed.

3. Change 11.9.3 [class.base.init] paragraph 8 as follows:

...An attempt to initialize more than one non-static data member of a union renders the program ill-formed. After the call to a constructor for class X has completed...

11.9.3 [class.base.init] paragraph 5 forbids initializing multiple members of a union via mem-initializers:

If a ctor-initializer specifies more than one mem-initializer for the same member, for the same base class or for multiple members of the same union (including members of anonymous unions), the ctor-initializer is ill-formed.

However, there is no corresponding restriction against specifying brace-or-equal-initializers for multiple union members, nor for a non-overlapping pair of brace-or-equal-initializer and mem-initializer. This is presumably an oversight.