On 11/27/19 6:35 PM, Marek Polacek wrote:
On Wed, Nov 27, 2019 at 04:47:01PM -0500, Jason Merrill wrote:
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?
We have a constructor that initializes a base class and members of a class:
{.D.2364={.i=12}, .a={.i=24}, .j=36}
Say we don't clear CONSTRUCTOR_NO_CLEARING in this ctor in
cxx_eval_call_expression.
Then soon in reduced_constant_expression_p we do
2221 field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
and since "Implement P0017R1, C++17 aggregates with bases. / r241187" we skip
base fields in C++17 so 'field' is set to 'a'.
Hmm?
next_initializable_field (tree field)
{
while (field
&& (TREE_CODE (field) != FIELD_DECL
|| DECL_UNNAMED_BIT_FIELD (field)
|| (DECL_ARTIFICIAL (field)
&& !(cxx_dialect >= cxx17 && DECL_FIELD_IS_BASE (field)))))
field = DECL_CHAIN (field);
This skips artificial fields except that in C++17 and up base fields are
*not* skipped.
How are you getting field starting with 'a'? Are you compiling in a
lower standard mode? The code using next_initializable_field doesn't
work for lower -std because of skipping base fields.
So perhaps we want to always clear_no_implicit_zero before c++20, and
always for c++20 and up?
Now we look at each element of the constructor; the first one is
D.2364={.i=12}. But we hit
2233 if (idx != field)
2234 return false;
because field is 'a' and idx is D.2364. Were you thinking of tweaking
reduced_constant_expression_p's behavior in C++20 and dropping the whole
cxx_eval_call_expression hunk?
--
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA
