[PATCH] D133659: [Clang] P1169R4: static operator()

[Roy Jacobson via Phabricator via cfe-commits]

royjacobson marked an inline comment as not done.
royjacobson 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">;
----------------
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?



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  https://reviews.llvm.org/D133659/new/

https://reviews.llvm.org/D133659