[PATCH] D156596: [Clang] Produce a warning instead of an error in unevaluated strings before C++26

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/include/clang/Basic/DiagnosticLexKinds.td:290
+  "encoding prefix '%0' on an unevaluated string literal has no effect"
+  "%select{| and is incompatible with c++2c}1">,
+  InGroup<DiagGroup<"invalid-unevaluated-string">>;
----------------
cor3ntin wrote:
> hubert.reinterpretcast wrote:
> > aaron.ballman wrote:
> > > hubert.reinterpretcast wrote:
> > > > I am not seeing any tests covering C mode.
> > > > 
> > > > @aaron.ballman, can you confirm that you are okay with the warning when processing C code (and, moreover, the newly-added errors for numeric escape sequences)?
> > > > 
> > > Good call on C test coverage, thank you for bringing that up!
> > > 
> > > I think the behavior in C is reasonable, even if it's not mandated by the standard. We're going from issuing an error in C to issuing a warning and the warning text helpfully omits the C++2c part of the diagnostic, so it seems like we're more relaxed in C than we previously were.
> > > 
> > > Do you have concerns about the behavior in C? (Or are you saying you'd prefer this to be an error in C as it was before?)
> > My question was with respect to the change from the Clang 16 status quo for numeric escapes introduced by the prior change and carried forward with this one: https://godbolt.org/z/aneGr1Yfb
> > 
> > ```
> > _Static_assert(1, "\x30"); // error in Clang 17
> > ```
> > 
> Yes, this is intended, numeric escape sequences in unevaluated string literals are not allowed in any language modes - the motivation for C is the same as C++. 
Barring evidence that we're breaking working code without workarounds, I'm happy with the behavior in C. I don't think numeric escape sequences make a lot of sense in the contexts where these strings appear in C, same as for C++. However, if we had evidence that this breaks working code in problematic ways, we could consider changing it to warning-defaults-to-error.


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  https://reviews.llvm.org/D156596/new/

https://reviews.llvm.org/D156596