Yes, I think we want an explicit group name (ideally the relevant library
or project name) because otherwise we just added another uncontrolled
global namespace to the C++ ecosystem. We can find existing practice in
user-defined attributes, which GCC supports some wildcarding for. And we
can see awkwardness in C++ module names that don't clearly define a
grouping semantic across module names, even ones that contain a '.'
character.

Failing a nice group name semantic for these messages, at best developers
will have to be disciplined to prefix their own diagnostic names
consistently the way that preprocessor macros and global symbols supposed
to be (and infamously aren't on occasion!). More likely, results will vary
and developers will step on one anothers' names. As a result, downstream
C++ projects will end up suffering.

Minimizing that sort of dysfunction at a distance is exactly what standards
are for.

But to reiterate, I like the idea and would like to see it implemented.

Bret

On Tue, Oct 15, 2024, 04:46 Corentin <corentin.jabot_at_[hidden]> wrote:

>
>
> On Tue, Oct 15, 2024 at 12:50 AM Bret Brown via SG15 <
> sg15_at_[hidden]> wrote:
>
>> I'm supportive of efforts to help library maintainers and users
>> communicate better, so I like it broadly.
>>
>> I would like to see some field describing the author of the diagnostic,
>> though.
>>
>> If `-Wall` was fully open to extension, I might need to disallow the
>> entire flag from my codebase, unfortunately. It's already sort of breaking
>> deprecation workflows that `-Werror=all` hard errors on any usage of `[[
>> deprecated ]]`. Given that erroring on every usage of a deprecated entity,
>> library authors might as well just delete the deprecated API. I anticipate
>> similar issues with warnings coming from libraries, especially on the
>> initial release of any toolchains that allow libraries to add -
>> `-Wall` warnings in particular.
>>
>> So I am thinking that it might be inappropriate to identify these issues
>> in the same category as toolchains provided diagnostics. Note that this
>> concept of diagnostic issues is modeled in interop standards, like SARIF
>> that was been discussed in SG-15 this summer. Each warning and error is
>> attributable to a specific "driver" like GCC or clang-tidy. So I would
>> expect each author of a given message would do well to self-identity using
>> the name of the relevant project. Then it would be clear that the project
>> "foobar" is emitting a warning and that the compiler is just the messenger.
>> As others in this thread have stated, users would likely like to be able to
>> enable, suppress, and prioritize in groups as well.
>>
>> Conceptually, we also should keep in mind that the `-W` flag has a
>> namespace and toolchains need to add more warnings to it as they see the
>> need. This is similar to how languages tend to reserve symbol names and
>> entire namespaced for themselves. If we allowed any arbitrary line of code
>> to change the quality of implementation of `-Wsomething`, then the
>> toolchain would have to figure out another way to express scope for built
>> in warnings names (A new flag? Namespaced warnings?). And existing users
>> (build system scripts and configurations) would be disrupted as they find
>> out that they needed to express whether they want the open-for-extension
>> `-Wsomething` or the one the toolchains supports and describes in its docs.
>>
>> I don't expect that to be well received. I predict instead that `-Wall`
>> wouldn't go anywhere. I could see some other flag being invented for this,
>> especially if it has some notion of scope so different libraries can
>> identify their warnings without worrying about warning name collisions.
>> Ideally library authors would scope warnings with their library name (since
>> they're providing diagnostics, not the compiler) and then an identifier. To
>> pick an arbitrary flag spelling (`-XYZ`) and an arbitrary library name
>> (`foobar`), something like `-XYZerror=foobar` or
>> `-XYZno-error=foobar::sizes-must-be-divisible-by-sixteen` could work.
>> Possibly even `-XYZerror=*::all`. And the driver in something like SARIF
>> could show "foobar" as the originator of the diagnostic.
>>
>
> [Implementer hat on]
>
> These wouldn't translate to some -Wfoo flag directly (even in WG21 asks
> nicely).
> Instead, it would be either a new flag, or some prefix like -Wuser-foo, or
> something that uses the tag in some way, at the discretion of the
> implementation
> And similarly with everything else, we'd most certainly support flags and
> pragma to disable such warnings and to turn them into errors on a per-tag
> basis - and I suspect we would introduce a group to disable/turn into error
> all such user defined warnings.
> The warning text would equally reflect that the warning is user-generated
> to limit the potential for abuse and confusion.
>
> Whether we want some limited support for user-provided diagnostic groups /
> some wildcard mechanism to affect multiple user-defined warnings at once
> is an interesting question. For this, to be remotely useful, users would
> have to be disciplined. Maybe if we require a group in all cases, we'd
> encourage discipline?
>
> You might find this llvm discussion interesting
> https://discourse.llvm.org/t/exposing-the-diagnostic-engine-to-c/73092/14
>
>
>
>
>
>> Anyway, tagging could be interesting, but I don't know that extending
>> existing `-Wall` would be popular. And I think identifying the driver or
>> provider or author (etc.) of the message would likely be more interesting
>> than the tagging feature. As a thought exercise, consider looking at some
>> example SARIF output from existing toolchains and consider how we would
>> expect one of these messages to appear as an entry. Then think what kind of
>> structures would enable the compiler to support that expectation.
>>
>
>> Bret
>>
>> On Mon, Oct 14, 2024, 17:44 Mathias Stearn via SG15 <
>> sg15_at_[hidden]> wrote:
>>
>>> You might want to allow some way to group these since compiler warnings
>>> are often turned on and off by groups in addition to specific flags. At the
>>> very least, it should be possible for each library to offer a group for all
>>> of its warnings, but huge libraries may want their own sub groups as well.
>>> One way would be to offer an overload that takes an initializer_list of
>>> tags so you could do
>>>
>>> std::constexpr_warning({"all", "format", "format-too-many-args"},
>>> "Format string consumed {} arguments but {} were provided.", current_arg,
>>> total);
>>>
>>> Which means that any of -Wall, -Wformat, or -Wformat-too-many-args (with
>>> -W replaced with a clash-free prefix) would turn that on, but it could also
>>> be specifically disabled with -Wno-format-too-many-args.
>>>
>>> I could also imagine a lighter-weight way of specifying these by taking
>>> a space or comma separated list in the string. If you go that route,
>>> obviously you would need to allow space or comma.
>>>
>>>
>>> On Mon, Oct 14, 2024 at 10:43 PM Zach Laine via SG15 <
>>> sg15_at_[hidden]> wrote:
>>>
>>>> For the tag in particular, this seems like just the right kind of
>>>> limitation.
>>>>
>>>> Zach
>>>>
>>>> On Mon, Oct 14, 2024 at 3:05 PM Barry Revzin via SG15
>>>> <sg15_at_[hidden]> wrote:
>>>> >
>>>> > Hey Tooling Study Group,
>>>> >
>>>> > I have this paper, P2758 (latest currently:
>>>> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2758r3.html),
>>>> which proposes low-level utilities for emitting messages during constant
>>>> evaluation time.
>>>> >
>>>> > Those messages have three kinds (print, warning, and error) and can
>>>> also be tagged. The intent of the tagging is to give the user the kind of
>>>> control typically reserved for the compiler. That is, the format library
>>>> can diagnose something with:
>>>> >
>>>> > std::constexpr_warning("format-too-many-args", "Format string
>>>> consumed {} arguments but {} were provided.", current_arg, total);
>>>> >
>>>> > And that'll emit a compiler warning that maybe can be explicitly
>>>> enabled (with some flag like -Wformat-too-many-args) or disabled (with some
>>>> flag like -Wno-format-too-many-args). And possibly likewise with #pragmas
>>>> for local blocks. Of course the actual mechanism is implementation-defined
>>>> and it's likely the flags won't be exactly that so that they won't clash
>>>> with actual implementation warnings.
>>>> >
>>>> > Evolution was happy with this proposal, but wanted you all to take a
>>>> look at it for its use of tagging to make sure that this is a viable path.
>>>> Right now, the paper's restriction on tagging is that it only contains,
>>>> basically, a-z, A-Z, 0-9, an underscore, or a hyphen — although it
>>>> presently also allows empty strings, which I'll change in a subsequent
>>>> revision. That restriction avoids having to really deal with unicode stuff,
>>>> while also matching the set of characters currently used in compiler flags
>>>> anyway, so doesn't seem like it's cutting off anything useful to me.
>>>> >
>>>> > Thanks in advance for the feedback,
>>>> >
>>>> > Barry
>>>> > _______________________________________________
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