From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>,
libstdc++ <libstdc++@gcc.gnu.org>,
Jonathan Wakely <jwakely@redhat.com>
Subject: Re: [PATCH] c++: Implement __is_{nothrow_,}convertible [PR106784]
Date: Fri, 23 Sep 2022 11:54:53 -0400 [thread overview]
Message-ID: <bcddca3e-b61d-730d-ac07-a33f6ba7fd38@redhat.com> (raw)
In-Reply-To: <Yy3D5NGL2kCCzmTx@redhat.com>
On 9/23/22 10:34, Marek Polacek wrote:
> On Thu, Sep 22, 2022 at 06:14:44PM -0400, Jason Merrill wrote:
>> On 9/22/22 09:39, Marek Polacek wrote:
>>> To improve compile times, the C++ library could use compiler built-ins
>>> rather than implementing std::is_convertible (and _nothrow) as class
>>> templates. This patch adds the built-ins. We already have
>>> __is_constructible and __is_assignable, and the nothrow forms of those.
>>>
>>> Microsoft (and clang, for compatibility) also provide an alias called
>>> __is_convertible_to. I did not add it, but it would be trivial to do
>>> so.
>>>
>>> I noticed that our __is_assignable doesn't implement the "Access checks
>>> are performed as if from a context unrelated to either type" requirement,
>>> therefore std::is_assignable / __is_assignable give two different results
>>> here:
>>>
>>> class S {
>>> operator int();
>>> friend void g(); // #1
>>> };
>>>
>>> void
>>> g ()
>>> {
>>> // #1 doesn't matter
>>> static_assert(std::is_assignable<int&, S>::value, "");
>>> static_assert(__is_assignable(int&, S), "");
>>> }
>>>
>>> This is not a problem if __is_assignable is not meant to be used by
>>> the users.
>>
>> That's fine, it's not.
>
> Okay then. libstdc++ needs to make sure then that it's handled right.
>
>>> This patch doesn't make libstdc++ use the new built-ins, but I had to
>>> rename a class otherwise its name would clash with the new built-in.
>>
>> Sigh, that's going to be a hassle when comparing compiler versions on
>> preprocessed code.
>
> Yeah, I guess :/. Kind of like __integer_pack / __make_integer_seq.
>
>>> --- a/gcc/cp/constraint.cc
>>> +++ b/gcc/cp/constraint.cc
>>> @@ -3697,6 +3697,12 @@ diagnose_trait_expr (tree expr, tree args)
>>> case CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS:
>>> inform (loc, " %qT does not have unique object representations", t1);
>>> break;
>>> + case CPTK_IS_CONVERTIBLE:
>>> + inform (loc, " %qT is not convertible from %qE", t2, t1);
>>> + break;
>>> + case CPTK_IS_NOTHROW_CONVERTIBLE:
>>> + inform (loc, " %qT is not %<nothrow%> convertible from %qE", t2, t1);
>>
>> It's odd that the existing diagnostics quote "nothrow", which is not a
>> keyword. I wonder why these library traits didn't use "noexcept"?
>
> Eh, yeah, only "throw" is. The quotes were deliberately added in
> <https://gcc.gnu.org/pipermail/gcc-patches/2019-May/522333.html>. Should
> I prepare a separate patch to use "%<noexcept%>" rather than "%<nothrow%>"?
> OTOH, the traits have "nothrow" in their names, so maybe just go back to
> "nothrow"?
The latter, I think. Or possibly "no-throw". I guess -Wformat-diag
wants "nothrow" quoted because of the attribute of that name.
>>> --- a/gcc/cp/method.cc
>>> +++ b/gcc/cp/method.cc
>>> @@ -2236,6 +2236,37 @@ ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
>>> return ref_conv_binds_directly (to, val, direct_init_p).is_false ();
>>> }
>>> +/* Return true if FROM can be converted to TO using implicit conversions,
>>> + or both FROM and TO are possibly cv-qualified void. NB: This doesn't
>>> + implement the "Access checks are performed as if from a context unrelated
>>> + to either type" restriction. */
>>> +
>>> +bool
>>> +is_convertible (tree from, tree to)
>>
>> You didn't want to add conversion to is*_xible?
>
> No, it didn't look like a good fit. It does things we don't need, and
> also has if VOID_TYPE_P -> return error_mark_node; which would be wrong
> for __is_convertible.
>
> I realized I'm not testing passing an incomplete type to the built-in,
> but since that is UB, I reckon we don't need to test it (we issue
> "error: invalid use of incomplete type").
But your patch does test that, in the existing call to check_trait_type
from finish_trait_expr?
The patch is OK.
Jason
next prev parent reply other threads:[~2022-09-23 15:54 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-22 13:39 Marek Polacek
2022-09-22 22:14 ` Jason Merrill
2022-09-23 14:34 ` Marek Polacek
2022-09-23 14:43 ` Jonathan Wakely
2022-09-23 16:34 ` Jonathan Wakely
2022-09-23 16:37 ` Marek Polacek
2022-09-23 15:54 ` Jason Merrill [this message]
2022-09-23 16:16 ` Marek Polacek
2022-09-23 14:40 ` Jonathan Wakely
2022-09-23 15:04 ` Marek Polacek
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