[PATCH] D124690: [clangd] add inlay hints for std::forward-ed parameter packs

Thu Jun 30 14:23:24 PDT 2022

upsj added inline comments.

================
Comment at: clang-tools-extra/clangd/InlayHints.cpp:483
+           !Type.getNonReferenceType().isConstQualified() &&
+           !isExpandedParameterPack(Param);
   }
----------------
sammccall wrote:
> upsj wrote:
> > sammccall wrote:
> > > upsj wrote:
> > > > sammccall wrote:
> > > > > sammccall wrote:
> > > > > > nridge wrote:
> > > > > > > sammccall wrote:
> > > > > > > > why is this check needed if we already decline to provide a name for the parameter on line 534 in chooseParameterNames?
> > > > > > > `shouldHintName` and `shouldHintReference` are [two independent conditions](https://searchfox.org/llvm/rev/508eb41d82ca956c30950d9a16b522a29aeeb333/clang-tools-extra/clangd/InlayHints.cpp#411-418) governing whether we show the parameter name and/or a `&` indicating pass-by-mutable-ref, respectively
> > > > > > > 
> > > > > > > (I did approve the [patch](https://reviews.llvm.org/D124359) that introduced `shouldHintReference` myself, hope that's ok)
> > > > > > Thanks, that makes sense! I just hadn't understood that change.
> > > > > What exactly *is* the motivation for suppressing reference hints in the pack case?
> > > > > 
> > > > > (I can imagine there are cases where they're annoying, but it's hard to know if the condition is right without knowing what those are)
> > > > I added an explanation. Basically, if we are unable to figure out which parameter the arguments are being forwarded to, the type of the ParmVarDecl for `Args&&...` gets deduced as `T&` or `T&&`, so that would mean even though we don't know whether the argument will eventually be forwarded to a reference parameter, we still claim all mutable lvalue arguments will be mutated, which IMO introduces more noise than necessary. But I think there are also good arguments for adding them to be safe.
> > > > 
> > > > There is another detail here, which is that we don't record whether we used std::forward, so the corresponding rvalue-to-lvalue conversions may lead to some unnecessary & annotations for rvalue arguments.
> > > This makes sense, the comment explains well, thank you!
> > > I have a couple of quibbles, up to you whether to change the logic.
> > > 
> > > #1: There's an unstated assumption that pack arguments *will* be forwarded (there are other things we can do with them, like use them in fold-expressions). It's a pretty good assumption but if the comment talks about forwarding, it should probably mention explicitly ("it's likely the params will be somehow forwarded, and...")
> > > 
> > > #2: the idea is that if the reference-ness is deduced from the callsite, then it's not meaningful as an "is the param modified" signal, it's just "is this arg modifiable". Fair enough, but this is a property of universal/forwarding references (T&& where T is a template param), not of packs. So I *think* this check should rather be !isInstantiatedFromForwardingReference(Param).
> > > But maybe that's more complexity and what you have is a good heuristic - I think at least we should call out that it's a heuristic for the true condition.
> > > 
> > > 
> > #1: I agree, I'll make that more clear before committing.
> > 
> > #2: Now that I think about it, there are actually two things we don't keep track of: parameters could lose their reference-ness via `Args...` instead of `Args&&...` and their rvalue-ness by not using `std::forward`. We only look at whether the innermost call takes a reference parameter, but as I said, we may lose some of that information on the way, claiming the function may modify the argument when it actually creates a copy on the way (losing reference-ness). I think the case of an rvalue being mistaken for an lvalue should not be much of an issue, since the reference annotation almost makes sense.
> > 
> > To visualize the situation: These three snippets all add &: hints to the parameter of bar
> > ```
> > void foo(int&);
> > template <typename... Args>
> > void bar(Args... args) { return foo(args...); }
> > void baz() {
> >   bar(1);
> > }
> > ```
> > ```
> > void foo(int&);
> > template <typename... Args>
> > void bar(Args&&... args) { return foo(args...); }
> > void baz() {
> >   bar(1);
> > }
> > ```
> > ```
> > void foo(int&);
> > template <typename... Args>
> > void bar(Args&&... args) { return foo(std::forward<Args>(args)...); }
> > void baz() {
> >   int a;
> >   bar(a);
> > }
> > ```
> > Two of these three cases probably shouldn't have this annotation?
> > parameters could lose their reference-ness via Args... instead of Args&&...
> (I'm not quite following what you mean here: if we deduce as `Args` rather than `Args&&` then the parameters are not references in the first place, we're passing by value)
> 
> > and their rvalue-ness by not using std::forward
> Yes. Fundamentally if we're deducing the ref type then we should be looking for a concrete signal of how the value is ultimately used, which involves tracking casts like std::forward. This is true whether it's a pack or not.
> It's a bunch of work and I'm not sure it's worth it (and I'm certainly not asking you to add it!)
> 
> > Two of these three cases probably shouldn't have this annotation?
> Yes. Claiming we're passing a numeric literal (prvalue) by mutable reference doesn't pass the laugh test.
> 
> > We only look at whether the innermost call takes a reference parameter, but as I said, we may lose some of that information on the way, claiming the function may modify the argument when it actually creates a copy on the way (losing reference-ness)
> 
> Right, there can be copies on the way for various reasons (passing by value, but also e.g. forwarding a T as a const U& where U is constructible from T).
> 
> I'm starting to think the simplest answer for now is never to include `&` if there's a forwarding T&& reference involved, as it's not always clear what it means and it's hard to do reliably.
> 
> This means we're accepting that `make_unique<Foo>(RefParamToConstructor)` will lack its `&`. (It already does today - this patch would need extensions to do this robustly).
> 
> I think we only need to check the outer call in the usual way, not any inner forwarded calls:
>  - if we're passing as (deduced) T then that's by value and no `&` is needed
>  - if we're passing as (deduced) T& then that's explicitly by reference (will only bind to a mutable lvalue) and `&` is needed
>  - if we're passing as (deduced) T&& then we're going to conservatively not include the `&`
> 
> I don't think packs need to be involved in this logic at all.
I //think// I found a suitable proxy for figuring out whether the function uses `Args&&...` and/or `std::forward`, can you take a final look at `shouldHintReference` whether this makes sense to you or I may have missed any important edge cases? I also added a few tests for this behavior (`VariadicReferenceHint...`) that should show that it works.

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  https://reviews.llvm.org/D124690/new/

https://reviews.llvm.org/D124690