On Tue, May 29, 2018 at 10:26 AM, Jakub Jelinek <ja...@redhat.com> wrote:
> On Tue, May 29, 2018 at 10:16:49AM -0400, Jason Merrill wrote:
>> On Tue, May 29, 2018, 4:31 AM Jakub Jelinek <ja...@redhat.com> wrote:
>> > Initializing the decomposition temporary from an expression with array type
>> > is a special aggregate initialization path in which we wouldn't mark the
>> > expression as read for the purposes of -Wunused-but-set*.
>> >
>> > Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for
>> > trunk?
>> >
>> > 2018-05-29 Jakub Jelinek <ja...@redhat.com>
>> >
>> > PR c++/85952
>> > * init.c (build_aggr_init): For structured binding initialized from
>> > array call mark_rvalue_use on the initializer.
>> >
>> > * g++.dg/warn/Wunused-var-33.C: New test.
>> >
>> > --- gcc/cp/init.c.jj 2018-05-25 14:34:41.000000000 +0200
>> > +++ gcc/cp/init.c 2018-05-28 19:04:10.504063972 +0200
>> > @@ -1678,6 +1678,7 @@ build_aggr_init (tree exp, tree init, in
>> > if (VAR_P (exp) && DECL_DECOMPOSITION_P (exp))
>> > {
>> > from_array = 1;
>> > + init = mark_rvalue_use (init);
>>
>> This should be mark_lvalue_use, since the structured bindings refer to
>> the elements of the array rather than copying them. OK with that
>> change.
>
> I think they refer to the elements of the decomposition variable (i.e. exp).
>
> "If the assignment-expression in the initializer has array type A and no
> ref-qualifier is present, e has type cv A and each element is
> copy-initialized or direct-initialized from the
> corresponding element of the assignment-expression as specified by the form
> of the
> initializer."
>
> is what applies in this case, and
>
> int a[2] = {1, 2};
> int D.2131[2] = a;
> int x [value-expr: D.2131[0]];
> int y [value-expr: D.2131[1]];
>
> <<cleanup_point int a[2] = {1, 2};>>;
> int D.2131[2] = a;
> return <retval> = x + y;
>
> is what original dump shows as implemented, so I don't see a being used here
> as an lvalue, we copy the elements into the temporary and that is all where
> it is referenced.
Ah, no, you're right for foo, where the structured binding declaration
is not a reference. And it looks like we shouldn't hit this path for
auto & [x,y] = a;
but that should be added to the testcase.
Jason
