On Mon, Feb 18, 2019 at 03:01:14PM -1000, Jason Merrill wrote:
> But it's not clear to me that the standard actually allows this. I don't
> think changing the active member of a union in the mem-initializer for
> another member is reasonable.
There is in [expr.const]/2:
an lvalue-to-rvalue conversion (7.1) that is applied to a glvalue that refers
to a non-active member of a
union or a subobject thereof;
an assignment expression (8.18) or invocation of an assignment operator (15.8)
that would change the
active member of a union;
in C++17 it seems, so maybe my union testcases are accepts-invalid.
This has been introduced in P0137R1 and removed again in P1330R0. Does that
mean e.g. following is valid in C++14, invalid in C++17 and valid again in
C++20? Or has one of the above papers changed retroactively previous
standards?
// PR c++/89336
// { dg-do compile { target c++14 } }
constexpr int
foo ()
{
union U { int a; long b; };
union V { union U u; short v; };
V w {};
w.u.a = w.v = w.u.b = 5L;
return w.u.a;
}
static_assert (foo () == 5, "");
constexpr int
bar ()
{
union U { int a[5]; long b; };
union V { union U u; short v; };
V w {};
w.v = 5;
w.u.a[3] = w.u.a[1] = w.v;
return w.u.a[1] + w.u.a[3];
}
static_assert (bar () == 10, "");
struct Z { int x, y; };
constexpr Z
baz ()
{
union W { Z a; long long w; };
W w {};
w.a = { 5, 0 };
w.a = { (int) (w.w = 17LL + w.a.x), 2 };
return w.a;
}
static_assert (baz ().x == 22, "");
static_assert (baz ().y == 2, "");
Jakub
