On Tue, Feb 19, 2019 at 11:03:05PM +0100, Jakub Jelinek wrote:
> > Before P0137 I believe foo and bar were arguably undefined.
>
> I see, before that it was:
> "an lvalue-to-rvalue conversion (4.1) or modification (5.18, 5.2.6, 5.3.2)
> that is applied
> to a glvalue that refers to a non-active member of a union or a subobject
> thereof;"
>
> So, do we want something like this then (or free all vectors immediately
> there and return immediately)?
Bootstrapped/regtested now on x86_64-linux and i686-linux.
> 2019-02-19 Jakub Jelinek <ja...@redhat.com>
>
> PR c++/89336
> * constexpr.c (cxx_eval_store_expression): Diagnose changing of active
> union member for -std=c++17 and earlier.
>
> * g++.dg/cpp1y/constexpr-89336-3.C: New test.
>
> --- gcc/cp/constexpr.c.jj 2019-02-19 10:26:09.567962395 +0100
> +++ gcc/cp/constexpr.c 2019-02-19 22:53:45.702983814 +0100
> @@ -3890,8 +3890,20 @@ cxx_eval_store_expression (const constex
>
> if (code == UNION_TYPE && CONSTRUCTOR_NELTS (*valp)
> && CONSTRUCTOR_ELT (*valp, 0)->index != index)
> - /* Changing active member. */
> - vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
> + {
> + if (cxx_dialect < cxx2a)
> + {
> + if (!ctx->quiet)
> + error_at (cp_expr_loc_or_loc (t, input_location),
> + "change of the active member of a union "
> + "from %qD to %qD",
> + CONSTRUCTOR_ELT (*valp, 0)->index,
> + index);
> + *non_constant_p = true;
> + }
> + /* Changing active member. */
> + vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
> + }
>
> for (idx = 0;
> vec_safe_iterate (CONSTRUCTOR_ELTS (*valp), idx, &cep);
> --- gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C.jj 2019-02-19
> 14:13:19.990627715 +0100
> +++ gcc/testsuite/g++.dg/cpp1y/constexpr-89336-3.C 2019-02-19
> 22:59:02.294799026 +0100
> @@ -0,0 +1,46 @@
> +// 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; // { dg-error "change of the active
> member of a union from" "" { target c++17_down } }
> + return w.u.a;
> +}
> +
> +static_assert (foo () == 5, ""); // { dg-error "non-constant condition
> for static assertion" "" { target c++17_down } }
> + // { dg-message "expansion of" "" {
> target c++17_down } .-1 }
> +
> +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; // { dg-error "change of the active
> member of a union from" "" { target c++17_down } }
> + return w.u.a[1] + w.u.a[3];
> +}
> +
> +static_assert (bar () == 10, ""); // { dg-error "non-constant condition
> for static assertion" "" { target c++17_down } }
> + // { dg-message "expansion of" "" {
> target c++17_down } .-1 }
> +
> +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 }; // { dg-error "change of the
> active member of a union from" "" { target c++17_down } }
> + return w.a;
> +}
> +
> +static_assert (baz ().x == 22, ""); // { dg-error "non-constant condition
> for static assertion" "" { target c++17_down } }
> + // { dg-message "expansion of" "" {
> target c++17_down } .-1 }
> +static_assert (baz ().y == 2, ""); // { dg-error "non-constant condition
> for static assertion" "" { target c++17_down } }
> + // { dg-message "expansion of" "" {
> target c++17_down } .-1 }
>
Jakub
