Full name of submitter (unless configured in github; will be published with the issue): Jiang An

Reference (section label): [class.copy.ctor], [class.copy.assign]

Link to reflector thread (if any):

Issue description:

It's specified in [class.copy.ctor] p15 and [class.copy.assign] p13 that a trivial and elegible copy or move function of a union copies the object representation, which can be problematic when the union object is a potentially-overlapping member subobject, and another subobject of the complete object overlaps with its tail padding, because the latter will be unintentionally overwritten.

Suggested resolution:

Change [class.copy.ctor] p15 as indicated:

The implicitly-defined copy/move constructor for a union X copies the object value representation ([basic.types.general]) of X. [...]

Change [class.copy.assign] p13 as indicated:

The implicitly-defined copy/move assignment operator for a union X copies the object value representation ([basic.types.general]) of X. [...]