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

[Tobias Ribizel via Phabricator via cfe-commits]

upsj marked an inline comment as done.
upsj added a comment.

yes, I have commit access



================
Comment at: clang-tools-extra/clangd/InlayHints.cpp:483
+           !Type.getNonReferenceType().isConstQualified() &&
+           !isExpandedParameterPack(Param);
   }
----------------
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?


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

https://reviews.llvm.org/D124690