[cfe-dev] Challenging "-Wctad-maybe-unsupported" for 9.0.0

[Arthur O'Dwyer via cfe-dev]

On Sun, Aug 4, 2019 at 5:25 AM JVApen <JVApen at gmail.com> wrote:

> On Sun, 4 Aug 2019 at 00:37, Eric Fiselier <eric at efcs.ca> wrote:
>
>> On Sat, Aug 3, 2019 at 10:14 AM Arthur O'Dwyer via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> On Sat, Aug 3, 2019 at 6:03 AM JVApen via cfe-dev <
>>> cfe-dev at lists.llvm.org> wrote:
>>>
>>>> Hello all,
>>>>
>>>> I'm currently testing 9.0.0 and I'm one of those strange people that
>>>> uses -Weverything in production, as we only support a single clang-release
>>>> at the same time. (Actually clang-cl, as it's Windows-only)
>>>> During the upgrade, I've noticed a new warning
>>>> "-Wctad-maybe-unsupported".
>>>> At first, I was really happy seeing it and started updating some code
>>>> that implicitly used it.
>>>>
>>>
Rewinding a bit: JVApen, could you explain a bit about why you felt the
need to "update" code that used CTAD?
Also, you said "implicitly used" — is there any difference in your mind
between "implicitly using" CTAD and "explicitly using" CTAD? What would the
latter look like?

Over half of the updates I had to do were about `std::unique_lock`,
>>>> `std::scope_guard` ... (and this without actively using this feature).
>>>>
>>>
This implies that there were also some cases where people were using CTAD
on types that weren't unique_lock or [I assume you meant
scoped_lock/lock_guard, not scope_guard]. What's your opinion of those
cases? While updating the code, did you find any usages of CTAD that were
actually incorrect and/or unintentional?
How would you tell whether a use of CTAD was intentional or unintentional?


The situation for me is exactly analogous to VLAs in C — including the
>>> shape of the workaround: "write an explicit call to malloc()" versus "write
>>> an explicit call to make_foo()".
>>> https://reviews.llvm.org/D54565 implements a `-Wctad` warning analogous
>>> to `-Wvla`.
>>>
>>
> I'm not sure we are talking about the same thing here, from what I read in
> that review, it flags all usages of CTAD. While the warning I mentioned
> doesn't trigger if the deduction guide exists. Although both use-cases are
> relevant, I'm in the luxurious position of having a single C++ standard to
> work with (and worry about).
>

In my experience, the existence of a deduction guide (written by the
library programmer, reflecting only the extent of the library programmer's
imagination) has no bearing on whether any particular usage of CTAD
(written by the client programmer, often indicating a typo or thinko) is
correct or not.
https://godbolt.org/z/_EfVON

–Arthur
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