Proposed resolution (February, 2017):

The resolution of issue 2272 also resolves this issue.

Proposed resolution (November, 2016) [SUPERSEDED]:

Change 9.4.2 [dcl.init.aggr] paragraph 8 as follows:

If there are fewer initializer-clauses in the list than there are elements in the aggregate, then each Each non-variant element of the aggregate that is not explicitly initialized shall be is initialized from its default member initializer (11.4 [class.mem]) or, if there is no default member initializer, copy-initialized from an empty initializer list (9.4.5 [dcl.init.list]). If the aggregate is a union and the initializer list is empty, then

• if any union member has a default member initilizer, that member is initialized from its default member initializer;

• otherwise, the first member of the union (if any) is copy-initialized from an empty initializer list.

[Example:...

[Adopted at the February/March, 2017 meeting.]

According to 9.4.2 [dcl.init.aggr] paragraph 15,

When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer-clause for the first non-static data member of the union.

This would appear to preclude using {} as the initializer for a union, which would otherwise have reasonable semantics. Is there a reason for this restriction?

Also, paragraph 7 reads,

If there are fewer initializer-clauses in the list than there are members in the aggregate, then each member not explicitly initialized shall be initialized from an empty initializer list (9.4.5 [dcl.init.list]).

There should presumably be special treatment for unions, so that only a single member is initialized in such cases.

(See also issue 1460.)