https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95976
Bug ID: 95976
Summary: [[no_unique_address]] on union members has the
opposite-of-intended effect
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: metaprogrammingtheworld at gmail dot com
Target Milestone: ---
When using [[no_unique_address]] with union members, ironically, each member is
now given a unique address! Without [[no_unique_address]], the layout is as one
would expect. This is particularly unfortunate since it means that a union's
size grows in proportion to the number of members when using
[[no_unique_address]]!
Example:
//////////
struct empty {};
union no_attribute_t
{
empty _0;
empty _1;
};
union with_attribute_t
{
[[no_unique_address]] empty _0;
[[no_unique_address]] empty _1;
};
constexpr no_attribute_t no_attribute{};
constexpr with_attribute_t with_attribute{};
// This succeeds
static_assert( &no_attribute._0 == &no_attribute._1 );
// This fails
static_assert( &with_attribute._0 == &with_attribute._1 );
//////////
