On 11/27/19 2:36 PM, Marek Polacek wrote:
On Sun, Nov 24, 2019 at 12:24:48PM -0500, Jason Merrill wrote:
On 11/16/19 5:23 PM, Marek Polacek wrote:
[ Working virtually on Baker Island. ]
This patch implements C++20 P1331, allowing trivial default initialization in
constexpr contexts.
I used Jakub's patch from the PR which allowed uninitialized variables in
constexpr contexts. But the hard part was handling CONSTRUCTOR_NO_CLEARING
which is always cleared in cxx_eval_call_expression. We need to set it in
the case a constexpr constructor doesn't initialize all the members, so that
we can give proper diagnostic instead of value-initializing. A lot of my
attempts flopped but then I came up with this approach, which handles various
cases as tested in constexpr-init8.C, where S is initialized by a non-default
constexpr constructor, and constexpr-init9.C, using delegating constructors.
And the best part is that I didn't need any new cx_check_missing_mem_inits
calls! Just save the information whether a constructor is missing an init
into constexpr_fundef_table and retrieve it when needed.
Is it necessary to clear the flag for constructors that do happen to
initialize all the members? I would think that leaving that clearing to
reduced_constant_expression_p would be enough.
It seems so: if I tweak cxx_eval_call_expression to only call
clear_no_implicit_zero
when 'fun' isn't DECL_CONSTRUCTOR_P, then a lot breaks, e.g. constexpr-base.C
where the constructor initializes all the members. By breaking I mean spurious
errors coming from
5937 if (TREE_CODE (r) == CONSTRUCTOR && CONSTRUCTOR_NO_CLEARING (r))
5938 {
5939 if (!allow_non_constant)
5940 error ("%qE is not a constant expression because it refers to "
5941 "an incompletely initialized variable", t);
5942 TREE_CONSTANT (r) = false;
5943 non_constant_p = true;
5944 }
Why didn't reduced_constant_expression_p unset CONSTRUCTOR_NO_CLEARING?
constexpr-init10.C demonstrates that we can now elide a constructor call,
this is caused by the walk_field_subobs hunk. I hope that's OK.
So long as the object's vptr is initialized properly, absolutely. Since a
has static storage duration, constant zero-initialization plus the default
constructor fully initialize the object.
Ah, that's comforting to hear, thanks.
--
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA
