cor3ntin added inline comments.
================
Comment at: clang/include/clang/Basic/DiagnosticParseKinds.td:1037-1041
+def err_static_mutable_lambda : Error<
+ "lambda cannot be both mutable and static">;
+def err_static_lambda_captures : Error<
+ "a static lambda cannot have any captures">;
+def note_lambda_captures : Note<"captures declared here">;
----------------
royjacobson wrote:
> aaron.ballman wrote:
> > royjacobson wrote:
> > > aaron.ballman wrote:
> > > > royjacobson wrote:
> > > > > aaron.ballman wrote:
> > > > > > These are semantic errors, not parsing ones. This means these will
> > > > > > be diagnosed when parsing the lambda rather than when instantiating
> > > > > > it. I don't think that matters for the cast of combining `mutable`
> > > > > > and `static`, but I'm less certain about "have any captures"
> > > > > > because of cases like:
> > > > > > ```
> > > > > > template <typename... Types>
> > > > > > auto func(Types... Ts) {
> > > > > > return [Ts...] { return 1; };
> > > > > > }
> > > > > >
> > > > > > int main() {
> > > > > > auto lambda = func();
> > > > > > }
> > > > > > ```
> > > > > > I'm pretty sure that lambda has no captures for that call, but it
> > > > > > could have captures depending on the instantiation.
> > > > > >
> > > > > > Actually, from some off-list discussion with @erichkeane, even
> > > > > > mutable and static are a problem in code like:
> > > > > > ```
> > > > > > template <typename Ty>
> > > > > > void func(T t) {
> > > > > > if constexpr (Something<T>) {
> > > > > > [](){};
> > > > > > } else {
> > > > > > [t](){};
> > > > > > }
> > > > > > ```
> > > > > > where the lambda is in a discarded statement.
> > > > > >
> > > > > > So I think these might need to change to be Sema diagnostics (and
> > > > > > we should add some additional test coverage).
> > > > > From https://eel.is/c++draft/expr.prim.lambda.general#4
> > > > >
> > > > > > If the lambda-specifier-seq contains static, there shall be no
> > > > > > lambda-capture
> > > > >
> > > > > So this should be a parsing error. Or maybe I don't understand what
> > > > > you're saying. There are no static lambdas in your examples so I'm
> > > > > not sure how they're related.
> > > > >
> > > > > So this should be a parsing error. Or maybe I don't understand what
> > > > > you're saying.
> > > >
> > > > Parsing errors are where the grammar disallows something, generally.
> > > > The rest are semantic diagnostics (e.g., we can parse the construct
> > > > just fine, but we diagnose when turning it into an AST node because
> > > > that's the point at which we have complete information about what we've
> > > > parsed).
> > > >
> > > > That said, my concern was mostly around SFINAE situations. My
> > > > recollection is that SFINAE traps do not cover parsing errors only type
> > > > substitution errors. So for my first example, I would expect there to
> > > > be no parsing error despite specifying a capture list because that
> > > > capture list can be empty when the pack is empty, but we would get a
> > > > SFINAE diagnostic when rebuilding declaration during template
> > > > instantiation if the pack was not empty.
> > > >
> > > > > There are no static lambdas in your examples so I'm not sure how
> > > > > they're related.
> > > >
> > > > Sorry, I was being lazy with my examples and showing more about the
> > > > capture list. Consider:
> > > > ```
> > > > template <typename... Types>
> > > > auto func(Types... Ts) {
> > > > return [Ts...] static { return 1; };
> > > > }
> > > >
> > > > int main() {
> > > > auto lambda = func();
> > > > }
> > > > ```
> > > >
> > > Like I said,
> > >
> > > > If the lambda-specifier-seq contains static, there shall be no
> > > > lambda-capture
> > >
> > > `Ts...` is still a syntactic `lambda-capture`, even if it's instantiated
> > > as an empty pack. I don't see how that's not a parsing error.
> > Ah, I think I maybe see where the confusion is coming in, now: you think my
> > example should be diagnosed and I think my example should be accepted.
> >
> > Based on the standards wording, I think you're right. The standard
> > specifically uses "lambda-capture" as a grammar term, so it *is* a parsing
> > error at that point.
> >
> > Based on my understanding of the intent behind the feature, I think I'm
> > right and there's a core issue here. I don't think we intended to prohibit
> > empty packs from being captured as non-SFINAEable error (and in off-list
> > talks with @erichkeane, he agrees). as that allows different
> > specializations of the function containing the lambda, which could be of
> > use. However, I'm not 100% sure on the intent. CC @hubert.reinterpretcast
> > as the C++ standards conformance code owner to see if he has an opinion or
> > other recollections here.
> I can see something like
>
> ```
> [Ts...] static(sizeof...(Types) == 0) {}
> ```
>
> being eventually useful, but as long as the `static` is non-conditional
> allowing a capture list with an empty pack doesn't make sense to me. I think
> you can always do something like
> ```
> []<typename = std::enable_if<sizeof...(Types) == 0>::value> {}
> ```
> and that should have the same functionality.
>
> Do you or Erich plan to open a CWG issue about this?
>
FYI I agree with @royjacobson on the current wording interpretation.
I don't think sfinae on empty pack offer much benefit - versus the complexity
(and the cost of not being able to diagnose non-empty packs until
instanciations) so I don't think there is a core issue here either. But until
such core issue emerges and is resolved, I don't think we need to change what
is done here.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D133659/new/
https://reviews.llvm.org/D133659
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