On Tue, May 09, 2017 at 04:37:16PM -0400, Jason Merrill wrote:
> For C++17 aggregate bases, we have started adding base fields for
> empty bases. The code for calculating whether a class is standard
> layout needs to ignore these.
>
> The C++17 mode diagnostic for direct-enum-init1.C was incorrect.
>
> Tested x86_64-pc-linux-gnu, applying to trunk.
> commit 9a612cc30d4b3ef905ce45304545d8b99a3cf5b9
> Author: Jason Merrill <ja...@redhat.com>
> Date: Tue May 9 14:15:38 2017 -0400
>
> * class.c (check_bases): Ignore empty bases.
This should have referenced PR c++/80605 (and is also a 7 regression).
> diff --git a/gcc/cp/class.c b/gcc/cp/class.c
> index fc71766..085dbc3 100644
> --- a/gcc/cp/class.c
> +++ b/gcc/cp/class.c
> @@ -1860,7 +1860,9 @@ check_bases (tree t,
> members */
> for (basefield = TYPE_FIELDS (basetype); basefield;
> basefield = DECL_CHAIN (basefield))
> - if (TREE_CODE (basefield) == FIELD_DECL)
> + if (TREE_CODE (basefield) == FIELD_DECL
> + && DECL_SIZE (basefield)
> + && !integer_zerop (DECL_SIZE (basefield)))
Is that what we really want? I mean, shouldn't we at least also
check that the basefield we want to ignore is DECL_ARTIFICIAL,
or that it doesn't have DECL_NAME or something similar, to avoid
considering user fields with zero size the same?
I believe your change changes e.g.:
struct S { int a[0]; };
struct T : public S { int b[0]; int c; };
bool q = __is_standard_layout (T);
which previously e.g. with -std=gnu++14 emitted q = false, but
now emits q = true.
Jakub
