[PATCH] D95691: Implement P2173 for attributes on lambdas

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


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Comment at: clang/include/clang/Basic/DiagnosticGroups.td:269
+def CXXPre2BCompatPedantic :
+  DiagGroup<"c++98-c++11-c++14-c++17-c++20-compat-pedantic", [CXXPre2BCompat]>;
 
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rjmccall wrote:
> Quuxplusone wrote:
> > aaron.ballman wrote:
> > > rjmccall wrote:
> > > > Uh, I think we're a couple standard releases past the point at which we should have reconsidered this schema.  I guess the problem is that we can't say `-Wpre-c++23-compat` without jumping the gun.  Is there a problem with `-Wc++20-compat` and then having the earlier warning groups imply the later ones?  That seems to be what we do with `-Wc++98-compat`; did we abandon that approach intentionally?
> > > @rsmith may have more background here. I was following the pattern already in the file, but I tend to agree that this pattern is not leading us somewhere good. FWIW, I ran into a similar situation with this on the C side of things in D95396, so we should probably be consistent there too.
> > My understanding is that the //command-line user// is expected to pass
> > - `clang++ -std=c++20 -Wc++11-compat` to indicate "I want //actually// to compile in C++20 mode, but give me warnings about anything that would prevent compiling in C++11 mode"
> > - `clang++ -std=c++17 -Wc++14-compat` to indicate "I want //actually// to compile in C++17 mode, but give me warnings about anything that would prevent compiling in C++14 mode"
> > - `clang++ -std=c++14 -Wc++20-compat` to indicate "I want //actually// to compile in C++14 mode, but give me warnings about anything that would prevent compiling in C++20 mode" — EXCEPT that I think this is not supported. My impression is that forward-compatibility warnings are generally just rolled into `-Wall` and not handled separately beyond that?
> > 
> > I don't think any human user is expected to pass `-Wc++98-c++11-c++14-c++17-c++20-compat` by hand; it's just an internal name for a particular subset of `-Wc++98-compat`.
> > 
> > IOW, we could choose a new naming scheme for it, but that would be a purely internal change that won't affect how command-line users interact with Clang at all (for better and for worse).
> Diagnostic groups can both directly contain diagnostics and imply other diagnostic groups, so I don't think there's any reason to make a dedicated group just to contain the new diagnostics in e.g. `-Wc++14-compat` except to allow someone turn on those warnings separately.  And it does show up to users as the warning group under `-fdiagnostics-show-option` (which is the default).
@Quuxplusone's comment describes the intent. `-std=c++20 -Wc++14-compat` should give a more or less complete list of reasons why the code would not compile in C++14 (at least on the language side; we don't check for stdlib compatibility). The other direction -- `-std=c++11 -Wc++14-compat` -- is more of a best-effort check for things that we've seen cause problems in practice and can easily detect. (As a consequence, I don't think there's any subset/superset relation between `-Wc++X-compat` and `-Wc++Y-compat`.)

I'd be happy to see these groups renamed to `-Wpre-c++20-compat` or similar. Warning group synonyms are relatively cheap, so I wouldn't be worried about adding a `-Wpre-c++2b-compat` now and renaming it to `-Wpre-c++23-compat` flag later.

(As an aside, it'd be handy if there were some way to mark a `DiagGroup` as existing only for grouping purposes, so that we could avoid exposing a `-W` flag for cases where groups are added for internal reasons.)


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Comment at: clang/include/clang/Basic/DiagnosticGroups.td:1023
+// earlier C++ versions.
+def CXX2B : DiagGroup<"c++2b-extensions">;
+
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I think we generally use a lowercase letter here, so `CXX2b`. (For example, see the `C2x` group.)


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https://reviews.llvm.org/D95691